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/schema2/config.py

@@ -213,6 +213,7 @@ class DockerSchema2Config(object):
         layer being last.
     """
     history = list(self.history)
+    assert layer_index < len(history)
 
     # If the layer is the leaf, it gets the full config (minus 2 fields). Otherwise, it gets only
     # IDs.