Container Support

The docker plugin has repository version uniqueness constraints for Tags. For example the Tag with name "latest" points to a Manifest List. Another Tag with name "latest" (pointing to another Manifest List) cannot be added to the repository version while the first "latest" tag remains. Note: This repository version could have a "latest" that points to a manifest instead of a manifest list.

At this point, we don't think this uniqueness constraint can exist in the db, so we need to enforce the constraint at sync time (and later during add/remove.

During sync, before the new tags are added to the repository version, corresponding tags that are already in the repository version should be removed.

Heres an example:
RepositoryVersion1 contains a tag, "latest" which points to ManifestList1. A sync retrieves a new Tag "latest" that points to ManifestList2. Before "latest"-> ManifestList2 is added to the repository version, "latest"->ManifestList1 must be removed.

For implementation, this probably should be a new Stage, which could be the last item in the pipeline. (Add/remove stages are added outside of the plugin-defined pipeline.)