Container manifests can define docker registry

- Allows for each image to be pulled from a custom docker registry
- ContainerManifest can specify authentication for a private registry
- No breaking changes
Here's the discussion on how image IDs might be generated https://github.com/dotcloud/docker/issues/6959 - there's would still be the changes to client, registry, and docker daemon API to allow pulling an image by its ID, vs a tag.