Enhancements for Docker schema implementations in preparing for supporting schema 2 in the OCI model

This adds additional required properties and methods to the Docker schema interface to allow us to treat both schema1 and schema2 manifests and lists logically equivalent from the OCI mode perspective

image/docker/schema2/test/test_config.py

@@ -127,3 +127,5 @@ def test_valid_config():
     else:
       assert 'Hostname' not in v1_compat['container_config']
       assert v1_compat['container_config']['Cmd'] == history_entry.command
+
+  assert config.labels == {}