Fix bug with json.dumps changing the order of keys

We write the literal received JSON string into the image data, to ensure the checksums computed will always match

data/registry_model/manifestbuilder.py

@@ -122,7 +122,7 @@ class _ManifestBuilder(object):
       model.image.set_image_metadata(layer_id, namespace_name, repo_name,
                                      v1_metadata.get('created'),
                                      v1_metadata.get('comment'),
-                                     command, json.dumps(v1_metadata),
+                                     command, v1_metadata_string,
                                      parent=parent_image)
 
     # Save the changes to the builder.