Properly handle the empty layer when pushing schema 2 manifests

Docker doesn't send us the contents of this layer, so we are forced to synthesize it ourselves

image/docker/interfaces.py

@@ -82,6 +82,11 @@ class ManifestInterface(object):
         of manifest does not support labels. """
     pass
 
+  @abstractmethod
+  def get_requires_empty_layer_blob(self, content_retriever):
+    """ Whether this schema requires the special empty layer blob. """
+    pass
+
   @abstractmethod
   def unsigned(self):
     """ Returns an unsigned version of this manifest. """