Work harder to reset the state of the docker env on the build worker.

workers/dockerfilebuild.py

@@ -125,7 +125,9 @@ class DockerfileBuildContext(object):
                  (self._repo, self._tag_names))
 
   def __enter__(self):
+    self.__cleanup_containers()
     self.__evict_expired_images()
+    self.__cleanup()
     return self
 
   def __exit__(self, exc_type, value, traceback):
@@ -291,8 +293,6 @@ class DockerfileBuildContext(object):
       self._build_cl.remove_container(container['Id'])
 
   def __evict_expired_images(self):
-    self.__cleanup_containers()
-
     logger.debug('Cleaning images older than %s hours.', CACHE_EXPIRATION_PERIOD_HOURS)
     now = datetime.now()
     verify_removed = set()
@@ -333,6 +333,7 @@ class DockerfileBuildContext(object):
         self.image_id_to_cache_time[image_id] = now
 
       for tag in image['RepoTags']:
+        # TODO this is slightly wrong, replace it with util/dockerfileparse.py when merged
         tag_repo = tag.split(':')[0]
         if tag_repo != '<none>':
           if self.__is_repo_public(tag_repo):