Add method for retrieving a V1-compatible manifest for any manifest or manifest list

This is used to serve older clients that don't support the V2 schema format

image/docker/interfaces.py

@@ -74,3 +74,9 @@ class ManifestInterface(object):
 
     Returns None if there are no legacy images associated with the manifest.
     """
+
+  @abstractmethod
+  def get_v1_compatible_manifest(self, namespace_name, repo_name, tag_name, lookup_fn):
+    """ Returns the manifest that is compatible with V1, by virtue of being `amd64` and `linux`.
+        If none, returns None.
+    """