diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 1dfc1957..0056f4c3 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -5,23 +5,23 @@
 (And thank you particularly for coming to read the guidelines! :heart_eyes:)
 
 The repo2docker developer documentation is all rendered on our documentation website: [https://repo2docker.readthedocs.io](https://repo2docker.readthedocs.io).
-If you're here, you're probably looking for the [Contributing to repo2docker development](https://repo2docker.readthedocs.io/en/latest/contributing/contributing.html) page.
+If you're here, you're probably looking for the [Contributing to repo2docker development](https://repo2docker.readthedocs.io/en/latest/contribute/contributing.html) page.
 
 Please make sure you've read the following sections before opening an issue/pull request:
 
-- [Process for making a contribution](https://repo2docker.readthedocs.io/en/latest/contributing/contributing.html#process-for-making-a-contribution).
+- [Process for making a contribution](https://repo2docker.readthedocs.io/en/latest/contribute/contributing.html#process-for-making-a-contribution).
   - These steps talk you through choosing the right issue template (bug report or feature request) and making a change.
-- [Guidelines to getting a Pull Request merged](https://repo2docker.readthedocs.io/en/latest/contributing/contributing.html#guidelines-to-getting-a-pull-request-merged).
+- [Guidelines to getting a Pull Request merged](https://repo2docker.readthedocs.io/en/latest/contribute/contributing.html#guidelines-to-getting-a-pull-request-merged).
   - These are tips and tricks to help make your contribution as smooth as possible for you and for the repo2docker maintenance team.
 
 There are a few other pages to highlight:
 
-- [Our roadmap](https://repo2docker.readthedocs.io/en/latest/contributing/roadmap.html)
+- [Our roadmap](https://repo2docker.readthedocs.io/en/latest/contribute/roadmap.html)
   - We use the roadmap to develop a shared understanding of the project's vision and direction amongst the community of users, contributors, and maintainers.
     This is a great place to get a feel for what the maintainers are thinking about for the short, medium, and long term future of the project.
 - [Design of repo2docker](https://repo2docker.readthedocs.io/en/latest/design.html)
   - This page explains some of the design principles behind repo2docker.
     Its a good place to understand _why_ the team have made the decisions that they have along the way!
   - We absolutely encourage discussion around refactoring, updating or extending repo2docker, but please make sure that you've understood this page before opening an issue to discuss the change you'd like to propose.
-- [Common developer tasks and how-tos](https://repo2docker.readthedocs.io/en/latest/contributing/tasks.html)
+- [Common developer tasks and how-tos](https://repo2docker.readthedocs.io/en/latest/contribute/tasks.html)
   - Some notes on running tests, buildpack dependencies, creating a release, and keeping the pip files up to date.
diff --git a/docs/source/changelog.md b/docs/source/changelog.md
index dc415aca..c6d9b7b7 100644
--- a/docs/source/changelog.md
+++ b/docs/source/changelog.md
@@ -1,5 +1,80 @@
 # Changelog
 
+## Unreleased breaking changes
+
+`RBuildPack.runtime` previously returned the contents of `runtime.txt` as a string.
+It has been replaced by `BuildPack.runtime` which returns a tuple `(name, version, date)`.
+
+## 2025.08.0
+
+([full changelog](https://github.com/jupyterhub/repo2docker/compare/4da768765372c602c06606cb79d21a398fcc2987...6cf91e45d5b03f79e365cd82eda09d1178d03327))
+
+### API and Breaking Changes
+
+- Switch to using CLI for everything except running the container [#1421](https://github.com/jupyterhub/repo2docker/pull/1421) ([@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+- Require Python 3.9 to run repo2docker [#1411](https://github.com/jupyterhub/repo2docker/pull/1411) ([@manics](https://github.com/manics), [@minrk](https://github.com/minrk), [@yuvipanda](https://github.com/yuvipanda))
+- Shell out to `docker buildx build` to build images [#1402](https://github.com/jupyterhub/repo2docker/pull/1402) ([@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+- notebook 7 [#1363](https://github.com/jupyterhub/repo2docker/pull/1363) ([@minrk](https://github.com/minrk), [@manics](https://github.com/manics))
+- Major version bump node (18->20) and jupyterhub-singleuser (3->5) [#1359](https://github.com/jupyterhub/repo2docker/pull/1359) ([@consideRatio](https://github.com/consideRatio), [@yuvipanda](https://github.com/yuvipanda))
+
+### Enhancements made
+
+- add triplets for r versions 4.3, 4.4 [#1403](https://github.com/jupyterhub/repo2docker/pull/1403) ([@minrk](https://github.com/minrk), [@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics))
+- Upgrade Shiny server to 2024.12 [#1400](https://github.com/jupyterhub/repo2docker/pull/1400) ([@rgaiacs](https://github.com/rgaiacs), [@manics](https://github.com/manics))
+- Upgrade RStudio to 2024.12 [#1399](https://github.com/jupyterhub/repo2docker/pull/1399) ([@rgaiacs](https://github.com/rgaiacs), [@manics](https://github.com/manics))
+- Use REST APIs to resolve DOIs + cleanup dataverse provider [#1390](https://github.com/jupyterhub/repo2docker/pull/1390) ([@yuvipanda](https://github.com/yuvipanda), [@minrk](https://github.com/minrk), [@pdurbin](https://github.com/pdurbin))
+- exclude defaults channel by default [#1365](https://github.com/jupyterhub/repo2docker/pull/1365) ([@minrk](https://github.com/minrk), [@rgaiacs](https://github.com/rgaiacs))
+
+### Bugs fixed
+
+- [MRG] Disable bash trace output in conda activation script [#1425](https://github.com/jupyterhub/repo2docker/pull/1425) ([@mfisher87](https://github.com/mfisher87), [@manics](https://github.com/manics))
+- Simulate json output from docker buildx build [#1413](https://github.com/jupyterhub/repo2docker/pull/1413) ([@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+- `set -e` should not be set after conda environment is sourced [#1409](https://github.com/jupyterhub/repo2docker/pull/1409) ([@manics](https://github.com/manics), [@yuvipanda](https://github.com/yuvipanda))
+- Use self.log rather than logging module directly [#1378](https://github.com/jupyterhub/repo2docker/pull/1378) ([@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics))
+- julia_project.py: fix Pkg REPL api warning [#1376](https://github.com/jupyterhub/repo2docker/pull/1376) ([@fonsp](https://github.com/fonsp), [@GeorgianaElena](https://github.com/GeorgianaElena), [@agoose77](https://github.com/agoose77))
+- TarFile.add: don't add recursively [#1371](https://github.com/jupyterhub/repo2docker/pull/1371) ([@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+
+### Maintenance and upkeep improvements
+
+- Bump mamba from 2.0.5 to 2.1.0 [#1423](https://github.com/jupyterhub/repo2docker/pull/1423) ([@weiji14](https://github.com/weiji14), [@yuvipanda](https://github.com/yuvipanda))
+- maint: Adapt mamba activation [#1419](https://github.com/jupyterhub/repo2docker/pull/1419) ([@jjerphan](https://github.com/jjerphan), [@yuvipanda](https://github.com/yuvipanda), [@minrk](https://github.com/minrk))
+- Add a test for runtime.txt with full R version specified [#1416](https://github.com/jupyterhub/repo2docker/pull/1416) ([@yuvipanda](https://github.com/yuvipanda), [@minrk](https://github.com/minrk))
+- Bump alpine docker to 3.21 [#1412](https://github.com/jupyterhub/repo2docker/pull/1412) ([@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+- Refreeze base environment for package upgrades [#1407](https://github.com/jupyterhub/repo2docker/pull/1407) ([@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics))
+- Remove unused \_urlopen method [#1392](https://github.com/jupyterhub/repo2docker/pull/1392) ([@yuvipanda](https://github.com/yuvipanda), [@minrk](https://github.com/minrk))
+- Upgraded to micromamba 2.0.5 [#1387](https://github.com/jupyterhub/repo2docker/pull/1387) ([@JohanMabille](https://github.com/JohanMabille), [@yuvipanda](https://github.com/yuvipanda), [@SylvainCorlay](https://github.com/SylvainCorlay))
+- Use self.log.warning instead of warnings.warn [#1384](https://github.com/jupyterhub/repo2docker/pull/1384) ([@yuvipanda](https://github.com/yuvipanda), [@manics](https://github.com/manics))
+- call close_handlers before garbage collection [#1380](https://github.com/jupyterhub/repo2docker/pull/1380) ([@minrk](https://github.com/minrk), [@manics](https://github.com/manics))
+- [pre-commit.ci] pre-commit autoupdate [#1377](https://github.com/jupyterhub/repo2docker/pull/1377) ([@minrk](https://github.com/minrk))
+- Update to mamba 1.5.9 [#1370](https://github.com/jupyterhub/repo2docker/pull/1370) ([@SylvainCorlay](https://github.com/SylvainCorlay), [@manics](https://github.com/manics))
+- Refreeze conda packages (JupyterLab 4.2.3 -> 4.2.5) [#1369](https://github.com/jupyterhub/repo2docker/pull/1369) ([@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+- Update for mamba 1.5.8 [#1367](https://github.com/jupyterhub/repo2docker/pull/1367) ([@jjerphan](https://github.com/jjerphan), [@manics](https://github.com/manics), [@SylvainCorlay](https://github.com/SylvainCorlay))
+- update import of shlex.quote [#1364](https://github.com/jupyterhub/repo2docker/pull/1364) ([@minrk](https://github.com/minrk), [@manics](https://github.com/manics))
+- [pre-commit.ci] pre-commit autoupdate [#1362](https://github.com/jupyterhub/repo2docker/pull/1362) ([@minrk](https://github.com/minrk))
+
+### Documentation improvements
+
+- Revamp our documentation and refactor a bit [#1433](https://github.com/jupyterhub/repo2docker/pull/1433) ([@choldgraf](https://github.com/choldgraf), [@chuckwondo](https://github.com/chuckwondo), [@rgaiacs](https://github.com/rgaiacs), [@yuvipanda](https://github.com/yuvipanda))
+- bug report.rst [#1429](https://github.com/jupyterhub/repo2docker/pull/1429) ([@nadiaguiffant](https://github.com/nadiaguiffant), [@manics](https://github.com/manics))
+- Add changelog for 2024.07.0 [#1356](https://github.com/jupyterhub/repo2docker/pull/1356) ([@yuvipanda](https://github.com/yuvipanda), [@consideRatio](https://github.com/consideRatio), [@manics](https://github.com/manics))
+
+### Continuous integration improvements
+
+- Add basic UI Playwright tests [#1410](https://github.com/jupyterhub/repo2docker/pull/1410) ([@manics](https://github.com/manics), [@minrk](https://github.com/minrk))
+- Don't run scheduled workflows on forks [#1408](https://github.com/jupyterhub/repo2docker/pull/1408) ([@manics](https://github.com/manics), [@yuvipanda](https://github.com/yuvipanda))
+- build(deps): bump codecov/codecov-action from 4 to 5 [#1381](https://github.com/jupyterhub/repo2docker/pull/1381) ([@minrk](https://github.com/minrk))
+- [pre-commit.ci] pre-commit autoupdate [#1358](https://github.com/jupyterhub/repo2docker/pull/1358) ([@minrk](https://github.com/minrk))
+- build(deps): bump docker/build-push-action from 5 to 6 [#1357](https://github.com/jupyterhub/repo2docker/pull/1357) ([@manics](https://github.com/manics))
+
+### Contributors to this release
+
+The following people contributed discussions, new ideas, code and documentation contributions, and review.
+See [our definition of contributors](https://github-activity.readthedocs.io/en/latest/#how-does-this-tool-define-contributions-in-the-reports).
+
+([GitHub contributors page for this release](https://github.com/jupyterhub/repo2docker/graphs/contributors?from=2024-07-01&to=2025-08-03&type=c))
+
+@agoose77 ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Aagoose77+updated%3A2024-07-01..2025-08-03&type=Issues)) | @AliMirlou ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3AAliMirlou+updated%3A2024-07-01..2025-08-03&type=Issues)) | @betatim ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Abetatim+updated%3A2024-07-01..2025-08-03&type=Issues)) | @choldgraf ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Acholdgraf+updated%3A2024-07-01..2025-08-03&type=Issues)) | @chuckwondo ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Achuckwondo+updated%3A2024-07-01..2025-08-03&type=Issues)) | @consideRatio ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3AconsideRatio+updated%3A2024-07-01..2025-08-03&type=Issues)) | @d70-t ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Ad70-t+updated%3A2024-07-01..2025-08-03&type=Issues)) | @felder ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Afelder+updated%3A2024-07-01..2025-08-03&type=Issues)) | @fonsp ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Afonsp+updated%3A2024-07-01..2025-08-03&type=Issues)) | @GeorgianaElena ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3AGeorgianaElena+updated%3A2024-07-01..2025-08-03&type=Issues)) | @Hind-M ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3AHind-M+updated%3A2024-07-01..2025-08-03&type=Issues)) | @hiroyuki-sato ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Ahiroyuki-sato+updated%3A2024-07-01..2025-08-03&type=Issues)) | @jjerphan ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Ajjerphan+updated%3A2024-07-01..2025-08-03&type=Issues)) | @JohanMabille ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3AJohanMabille+updated%3A2024-07-01..2025-08-03&type=Issues)) | @manics ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Amanics+updated%3A2024-07-01..2025-08-03&type=Issues)) | @mfisher87 ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Amfisher87+updated%3A2024-07-01..2025-08-03&type=Issues)) | @minrk ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Aminrk+updated%3A2024-07-01..2025-08-03&type=Issues)) | @nadiaguiffant ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Anadiaguiffant+updated%3A2024-07-01..2025-08-03&type=Issues)) | @pdurbin ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Apdurbin+updated%3A2024-07-01..2025-08-03&type=Issues)) | @rgaiacs ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Argaiacs+updated%3A2024-07-01..2025-08-03&type=Issues)) | @ryanlovett ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Aryanlovett+updated%3A2024-07-01..2025-08-03&type=Issues)) | @SylvainCorlay ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3ASylvainCorlay+updated%3A2024-07-01..2025-08-03&type=Issues)) | @weiji14 ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Aweiji14+updated%3A2024-07-01..2025-08-03&type=Issues)) | @yuvipanda ([activity](https://github.com/search?q=repo%3Ajupyterhub%2Frepo2docker+involves%3Ayuvipanda+updated%3A2024-07-01..2025-08-03&type=Issues))
+
 ## 2024.07.0
 
 ([full changelog](https://github.com/jupyterhub/repo2docker/compare/2024.03.0...2024.07.0))
diff --git a/repo2docker/app.py b/repo2docker/app.py
index b9684c11..7a70a470 100755
--- a/repo2docker/app.py
+++ b/repo2docker/app.py
@@ -733,7 +733,10 @@ class Repo2Docker(Application):
         try:
             self.fetch(self.repo, self.ref, checkout_path)
 
-            if self.find_image():
+            if self.find_image() and not self.push:
+                # If push is requested we don't have a general way to query the registry.
+                # ContainerEngine.build() also handles pushing, so always "build" and
+                # rely on the implementation to decide whether to rebuild.
                 self.log.info(
                     f"Reusing existing image ({self.output_image_spec}), not building."
                 )
diff --git a/repo2docker/buildpacks/base.py b/repo2docker/buildpacks/base.py
index f433029e..74bfa331 100644
--- a/repo2docker/buildpacks/base.py
+++ b/repo2docker/buildpacks/base.py
@@ -1,3 +1,4 @@
+import datetime
 import hashlib
 import io
 import logging
@@ -750,3 +751,50 @@ class BaseImage(BuildPack):
             # the only path evaluated at container start time rather than build time
             return os.path.join("${REPO_DIR}", start)
         return None
+
+    @property
+    def runtime(self):
+        """
+        Return parsed contents of runtime.txt
+
+        Returns (runtime, version, date), tuple components may be None.
+        Returns (None, None, None) if runtime.txt not found.
+
+        Supported formats:
+          name-version
+          name-version-yyyy-mm-dd
+          name-yyyy-mm-dd
+        """
+        if hasattr(self, "_runtime"):
+            return self._runtime
+
+        self._runtime = (None, None, None)
+
+        runtime_path = self.binder_path("runtime.txt")
+        try:
+            with open(runtime_path) as f:
+                runtime_txt = f.read().strip()
+        except FileNotFoundError:
+            return self._runtime
+
+        name = None
+        version = None
+        date = None
+
+        parts = runtime_txt.split("-")
+        if len(parts) not in (2, 4, 5) or any(not (p) for p in parts):
+            raise ValueError(f"Invalid runtime.txt: {runtime_txt}")
+
+        name = parts[0]
+
+        if len(parts) in (2, 5):
+            version = parts[1]
+
+        if len(parts) in (4, 5):
+            date = "-".join(parts[-3:])
+            if not re.match(r"\d\d\d\d-\d\d-\d\d", date):
+                raise ValueError(f"Invalid runtime.txt date: {date}")
+            date = datetime.datetime.fromisoformat(date).date()
+
+        self._runtime = (name, version, date)
+        return self._runtime
diff --git a/repo2docker/buildpacks/pipfile/__init__.py b/repo2docker/buildpacks/pipfile/__init__.py
index 2e24d614..da3f1fd3 100644
--- a/repo2docker/buildpacks/pipfile/__init__.py
+++ b/repo2docker/buildpacks/pipfile/__init__.py
@@ -187,12 +187,9 @@ class PipfileBuildPack(CondaBuildPack):
     def detect(self):
         """Check if current repo should be built with the Pipfile buildpack."""
         # first make sure python is not explicitly unwanted
-        runtime_txt = self.binder_path("runtime.txt")
-        if os.path.exists(runtime_txt):
-            with open(runtime_txt) as f:
-                runtime = f.read().strip()
-            if not runtime.startswith("python-"):
-                return False
+        name = self.runtime[0]
+        if name and name != "python":
+            return False
 
         pipfile = self.binder_path("Pipfile")
         pipfile_lock = self.binder_path("Pipfile.lock")
diff --git a/repo2docker/buildpacks/python/__init__.py b/repo2docker/buildpacks/python/__init__.py
index 1530e347..6c9a6d6e 100644
--- a/repo2docker/buildpacks/python/__init__.py
+++ b/repo2docker/buildpacks/python/__init__.py
@@ -15,14 +15,10 @@ class PythonBuildPack(CondaBuildPack):
         if hasattr(self, "_python_version"):
             return self._python_version
 
-        try:
-            with open(self.binder_path("runtime.txt")) as f:
-                runtime = f.read().strip()
-        except FileNotFoundError:
-            runtime = ""
+        name, version, _ = self.runtime
 
-        if not runtime.startswith("python-"):
-            # not a Python runtime (e.g. R, which subclasses this)
+        if name != "python" or not version:
+            # Either not specified, or not a Python runtime (e.g. R, which subclasses this)
             # use the default Python
             self._python_version = self.major_pythons["3"]
             self.log.warning(
@@ -30,7 +26,7 @@ class PythonBuildPack(CondaBuildPack):
             )
             return self._python_version
 
-        py_version_info = runtime.split("-", 1)[1].split(".")
+        py_version_info = version.split(".")
         py_version = ""
         if len(py_version_info) == 1:
             py_version = self.major_pythons[py_version_info[0]]
@@ -138,16 +134,11 @@ class PythonBuildPack(CondaBuildPack):
     def detect(self):
         """Check if current repo should be built with the Python buildpack."""
         requirements_txt = self.binder_path("requirements.txt")
-        runtime_txt = self.binder_path("runtime.txt")
         setup_py = "setup.py"
 
-        if os.path.exists(runtime_txt):
-            with open(runtime_txt) as f:
-                runtime = f.read().strip()
-            if runtime.startswith("python-"):
-                return True
-            else:
-                return False
+        name = self.runtime[0]
+        if name:
+            return name == "python"
         if not self.binder_dir and os.path.exists(setup_py):
             return True
         return os.path.exists(requirements_txt)
diff --git a/repo2docker/buildpacks/r.py b/repo2docker/buildpacks/r.py
index 556f6691..a10443fe 100644
--- a/repo2docker/buildpacks/r.py
+++ b/repo2docker/buildpacks/r.py
@@ -46,21 +46,6 @@ class RBuildPack(PythonBuildPack):
     R is installed from https://docs.rstudio.com/resources/install-r/
     """
 
-    @property
-    def runtime(self):
-        """
-        Return contents of runtime.txt if it exists, '' otherwise
-        """
-        if not hasattr(self, "_runtime"):
-            runtime_path = self.binder_path("runtime.txt")
-            try:
-                with open(runtime_path) as f:
-                    self._runtime = f.read().strip()
-            except FileNotFoundError:
-                self._runtime = ""
-
-        return self._runtime
-
     @property
     def r_version(self):
         """Detect the R version for a given `runtime.txt`
@@ -70,6 +55,7 @@ class RBuildPack(PythonBuildPack):
         """
         # Available versions at https://cran.r-project.org/src/base/
         version_map = {
+            "4.5": "4.5.1",
             "4.4": "4.4.2",
             "4.3": "4.3.3",
             "4.2": "4.2.3",
@@ -91,11 +77,11 @@ class RBuildPack(PythonBuildPack):
         r_version = version_map["4.4"]
 
         if not hasattr(self, "_r_version"):
-            parts = self.runtime.split("-")
+            _, version, date = self.runtime
             # If runtime.txt is not set, or if it isn't of the form r-<version>-<yyyy>-<mm>-<dd>,
             # we don't use any of it in determining r version and just use the default
-            if len(parts) == 5:
-                r_version = parts[1]
+            if version and date:
+                r_version = version
                 # For versions of form x.y, we want to explicitly provide x.y.z - latest patchlevel
                 # available. Users can however explicitly specify the full version to get something specific
                 if r_version in version_map:
@@ -131,15 +117,11 @@ class RBuildPack(PythonBuildPack):
         Returns '' if no date is specified
         """
         if not hasattr(self, "_checkpoint_date"):
-            match = re.match(r"r-(\d.\d(.\d)?-)?(\d\d\d\d)-(\d\d)-(\d\d)", self.runtime)
-            if not match:
-                self._checkpoint_date = False
+            name, version, date = self.runtime
+            if name == "r" and date:
+                self._checkpoint_date = date
             else:
-                # turn the last three groups of the match into a date
-                self._checkpoint_date = datetime.date(
-                    *[int(s) for s in match.groups()[-3:]]
-                )
-
+                self._checkpoint_date = False
         return self._checkpoint_date
 
     def detect(self):
@@ -157,13 +139,9 @@ class RBuildPack(PythonBuildPack):
 
         description_R = "DESCRIPTION"
         if not self.binder_dir and os.path.exists(description_R):
-            if not self.checkpoint_date:
-                # no R snapshot date set through runtime.txt
-                # Set it to two days ago from today
-                self._checkpoint_date = datetime.date.today() - datetime.timedelta(
-                    days=2
-                )
-                self._runtime = f"r-{str(self._checkpoint_date)}"
+            # no R snapshot date set through runtime.txt
+            # Set it to two days ago from today
+            self._checkpoint_date = datetime.date.today() - datetime.timedelta(days=2)
             return True
 
     @lru_cache
diff --git a/repo2docker/docker.py b/repo2docker/docker.py
index e6d3958c..984c074d 100644
--- a/repo2docker/docker.py
+++ b/repo2docker/docker.py
@@ -107,6 +107,13 @@ class DockerEngine(ContainerEngine):
     ):
         if not shutil.which("docker"):
             raise RuntimeError("The docker commandline client must be installed")
+
+        # docker buildx is based in a plugin that might not be installed
+        # https://github.com/docker/buildx
+        docker_buildx_version = subprocess.run(["docker", "buildx", "version"])
+        if docker_buildx_version.returncode:
+            raise RuntimeError("The docker buildx plugin must be installed")
+
         args = ["docker", "buildx", "build", "--progress", "plain"]
         if load:
             if push:
diff --git a/tests/r/r4.4-rspm/runtime.txt b/tests/r/r4.4-rspm/runtime.txt
deleted file mode 100644
index 1375871d..00000000
--- a/tests/r/r4.4-rspm/runtime.txt
+++ /dev/null
@@ -1 +0,0 @@
-r-4.4-2025-01-01
diff --git a/tests/r/r4.4-rspm/install.R b/tests/r/r4.5-rspm/install.R
similarity index 100%
rename from tests/r/r4.4-rspm/install.R
rename to tests/r/r4.5-rspm/install.R
diff --git a/tests/r/r4.5-rspm/runtime.txt b/tests/r/r4.5-rspm/runtime.txt
new file mode 100644
index 00000000..a5718197
--- /dev/null
+++ b/tests/r/r4.5-rspm/runtime.txt
@@ -0,0 +1 @@
+r-4.5.1-2025-06-13
diff --git a/tests/r/r4.4-rspm/verify b/tests/r/r4.5-rspm/verify
similarity index 65%
rename from tests/r/r4.4-rspm/verify
rename to tests/r/r4.5-rspm/verify
index 0e02bdd4..6d913061 100755
--- a/tests/r/r4.4-rspm/verify
+++ b/tests/r/r4.5-rspm/verify
@@ -1,9 +1,9 @@
 #!/usr/bin/env Rscript
 library('digest')
 
-# Fail if version is not 4.0
+# Fail if version is not 4.5
 print(version)
-if (!(version$major == "4" && as.double(version$minor) >= 4 && as.double(version$minor) < 5)) {
+if (!(version$major == "4" && as.double(version$minor) >= 5 && as.double(version$minor) < 6)) {
   quit("yes", 1)
 }
 
diff --git a/tests/unit/test_buildpack.py b/tests/unit/test_buildpack.py
index 38fc5c23..ce84df2f 100644
--- a/tests/unit/test_buildpack.py
+++ b/tests/unit/test_buildpack.py
@@ -1,9 +1,14 @@
+from datetime import date
 from os.path import join as pjoin
 from tempfile import TemporaryDirectory
 
 import pytest
 
-from repo2docker.buildpacks import LegacyBinderDockerBuildPack, PythonBuildPack
+from repo2docker.buildpacks import (
+    BaseImage,
+    LegacyBinderDockerBuildPack,
+    PythonBuildPack,
+)
 from repo2docker.utils import chdir
 
 
@@ -46,3 +51,45 @@ def test_unsupported_python(tmpdir, python_version, base_image):
     assert bp.python_version == python_version
     with pytest.raises(ValueError):
         bp.render()
+
+
+@pytest.mark.parametrize(
+    "runtime_txt, expected",
+    [
+        (None, (None, None, None)),
+        ("abc-001", ("abc", "001", None)),
+        ("abc-001-2025-06-22", ("abc", "001", date(2025, 6, 22))),
+        ("abc-2025-06-22", ("abc", None, date(2025, 6, 22))),
+        ("a_b/c-0.0.1-2025-06-22", ("a_b/c", "0.0.1", date(2025, 6, 22))),
+    ],
+)
+def test_runtime(tmpdir, runtime_txt, expected, base_image):
+    tmpdir.chdir()
+
+    if runtime_txt is not None:
+        with open("runtime.txt", "w") as f:
+            f.write(runtime_txt)
+
+    base = BaseImage(base_image)
+    assert base.runtime == expected
+
+
+@pytest.mark.parametrize(
+    "runtime_txt",
+    [
+        "",
+        "abc",
+        "abc-001-25-06-22",
+    ],
+)
+def test_invalid_runtime(tmpdir, runtime_txt, base_image):
+    tmpdir.chdir()
+
+    if runtime_txt is not None:
+        with open("runtime.txt", "w") as f:
+            f.write(runtime_txt)
+
+    base = BaseImage(base_image)
+
+    with pytest.raises(ValueError, match=r"^Invalid runtime.txt.*"):
+        base.runtime