Container Support

This user story is doable by the initial design (https://pulp.plan.io/issues/3405), but it currently requires users to make a number of preliminary calls to pulp to gather information that could be replaced with more arguments to a single call. My guess is that adding units by tag name and source repo will be a common use case.

This story is to accept repository version hrefs and tag names. If they are provided, they should be used to generate a list of tag content units to add to a repository, and this list should be passed to the rest of the work implemented in 3405.

Endpoint

POST v3/docker/tags/copy repo_source ='' repo_dest='' name=X will recursively copy everything under tag X
POST v3/docker/tags/copy repo_source ='' repo_dest='' will recursively copy all the tags