add assemble_subset

- get_assemble_files() returns list of files needed for assembly
- if buildpack.assemble_with_subset is set, only load assemble_files
  prior to running assembly scripts. Load the rest of the repo afterward

conda opts in to this, but currently I think this works
for everything *except* requirements.txt

repo2docker/buildpacks/conda/__init__.py

@@ -178,6 +178,18 @@ class CondaBuildPack(BaseImage):
         """Am I building a Python 2 kernel environment?"""
         return self.python_version and self.python_version.split(".")[0] == "2"
 
+    def get_assemble_files(self):
+        """Specify that assembly only requires environment.yml
+
+        enables caching assembly result even when
+        repo contents change
+        """
+        assemble_files = super().get_assemble_files()
+        environment_yml = self.binder_path('environment.yml')
+        if os.path.exists(environment_yml):
+            assemble_files.append(environment_yml)
+        return assemble_files
+
     def get_assemble_scripts(self):
         """Return series of build-steps specific to this source repository.
         """