feat: implement support for dockerignore and containerignore

Currently repo2docker creates a context object that includes the whole content
of the repository it builds an image for. Thus it includes folders like .git
which is usually something that has no interest in the final image, can take
quite a lot of space and most importantly, kills the caching of that layer.

This patch adds support for reading dockerignore and containerignore files that
are used to ensure only the relevant data are used to build the image.

By default it also excludes the .git folder if neither of these files are
provided.

repo2docker/buildpacks/base.py

@@ -12,6 +12,8 @@ from functools import lru_cache
 import escapism
 import jinja2
 
+from docker.utils.build import exclude_paths
+
 # Only use syntax features supported by Docker 17.09
 TEMPLATE = r"""
 FROM {{base_image}}
@@ -590,16 +592,16 @@ class BuildPack:
 
         tar.addfile(dockerfile_tarinfo, io.BytesIO(dockerfile))
 
-        def _filter_tar(tar):
+        def _filter_tar(tarinfo):
             # We need to unset these for build_script_files we copy into tar
             # Otherwise they seem to vary each time, preventing effective use
             # of the cache!
             # https://github.com/docker/docker-py/pull/1582 is related
-            tar.uname = ""
-            tar.gname = ""
-            tar.uid = int(build_args.get("NB_UID", DEFAULT_NB_UID))
-            tar.gid = int(build_args.get("NB_UID", DEFAULT_NB_UID))
-            return tar
+            tarinfo.uname = ""
+            tarinfo.gname = ""
+            tarinfo.uid = int(build_args.get("NB_UID", DEFAULT_NB_UID))
+            tarinfo.gid = int(build_args.get("NB_UID", DEFAULT_NB_UID))
+            return tarinfo
 
         for src in sorted(self.get_build_script_files()):
             dest_path, src_path = self.generate_build_context_filename(src)
@@ -608,7 +610,25 @@ class BuildPack:
         for fname in ("repo2docker-entrypoint", "python3-login"):
             tar.add(os.path.join(HERE, fname), fname, filter=_filter_tar)
 
-        tar.add(".", "src/", filter=_filter_tar)
+        exclude = []
+
+        for ignore_file in [".dockerignore", ".containerignore"]:
+            if os.path.exists(ignore_file):
+                with open(ignore_file, "r") as f:
+                    exclude.extend(
+                        list(
+                            filter(
+                                lambda x: x != "" and x[0] != "#",
+                                [l.strip() for l in f.read().splitlines()],
+                            )
+                        )
+                    )
+
+        if not exclude:
+            exclude = ["**/.git"]
+
+        for item in exclude_paths(".", exclude):
+            tar.add(item, f"src/{item}", filter=_filter_tar)
 
         tar.close()
         tarf.seek(0)