Docker Support

As a result of this story, Pulp Docker will support manifests lists for sync and publish.

To support manifest lists we need to:
1. Store the architecture, os, and any other values listed in the spec as an attribute of each manifest. The values are listed here under the "manifests" array:

https://docs.docker.com/registry/spec/manifest-v2-2/#manifest-list-field-descriptions

2. During sync always make 3 requests to the registry and in each request set properly the headers( We would need to make 3 requests to support schema 1, schema2 and manifest lists)
3. Change publish to create a manifest list
4. Change publish to create the new crane metadata file
5. Change the schema of the crane metadata file to have manifest lists for fat manifest also

This is an alternative to #2384