Allow specifying images to reuse cache from

This lets us explicitly specify images that repo2docker
should try to re-use cached layers from. Docker normally only
looks for layers from images that were *built* locally - if
we want it to look in images that were *pulled* from a registry,
we need to specify it here.

repo2docker/app.py

@@ -406,6 +406,13 @@ class Repo2Docker(Application):
             help='Print the repo2docker version and exit.'
         )
 
+        argparser.add_argument(
+            '--cache-from',
+            action='append',
+            default=[],
+            help='Docker images to attempt to re-use cached layers from'
+        )
+
         return argparser
 
     def json_excepthook(self, etype, evalue, traceback):
@@ -545,6 +552,9 @@ class Repo2Docker(Application):
         if args.subdir:
             self.subdir = args.subdir
 
+        if args.cache_from:
+            self.cache_from = args.cache_from
+
         self.environment = args.environment
 
     def push_image(self):
@@ -719,7 +729,7 @@ class Repo2Docker(Application):
                               extra=dict(phase='building'))
 
                 for l in picked_buildpack.build(self.output_image_spec,
-                    self.build_memory_limit, build_args):
+                    self.build_memory_limit, build_args, self.cache_from):
                     if 'stream' in l:
                         self.log.info(l['stream'],
                                       extra=dict(phase='building'))

repo2docker/buildpacks/base.py

@@ -449,7 +449,7 @@ class BuildPack:
             appendix=self.appendix,
         )
 
-    def build(self, image_spec, memory_limit, build_args):
+    def build(self, image_spec, memory_limit, build_args, cache_from):
         tarf = io.BytesIO()
         tar = tarfile.open(fileobj=tarf, mode='w')
         dockerfile_tarinfo = tarfile.TarInfo("Dockerfile")
@@ -499,7 +499,8 @@ class BuildPack:
                 decode=True,
                 forcerm=True,
                 rm=True,
-                container_limits=limits
+                container_limits=limits,
+                cache_from=cache_from
         ):
             yield line
 

repo2docker/buildpacks/docker.py

@@ -19,7 +19,7 @@ class DockerBuildPack(BuildPack):
         with open(Dockerfile) as f:
             return f.read()
 
-    def build(self, image_spec, memory_limit, build_args):
+    def build(self, image_spec, memory_limit, build_args, cache_from):
         """Build a Docker image based on the Dockerfile in the source repo."""
         limits = {
             # Always disable memory swap for building, since mostly
@@ -37,6 +37,7 @@ class DockerBuildPack(BuildPack):
                 decode=True,
                 forcerm=True,
                 rm=True,
-                container_limits=limits
+                container_limits=limits,
+                cache_from=cache_from
         ):
             yield line