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/schema2/config.py

@@ -205,6 +205,15 @@ class DockerSchema2Config(object):
     """ Returns a dictionary of all the labels defined in this configuration. """
     return self._parsed.get('config', {}).get('Labels', {}) or {}
 
+  @property
+  def has_empty_layer(self):
+    """ Returns whether this config contains an empty layer. """
+    for history_entry in self._parsed[DOCKER_SCHEMA2_CONFIG_HISTORY_KEY]:
+      if history_entry.get(DOCKER_SCHEMA2_CONFIG_EMPTY_LAYER_KEY, False):
+        return True
+    
+    return False
+
   @property
   def history(self):
     """ Returns the history of the image, started at the base layer. """