Arvados

It's handy to be able to copy a Docker image collection from one project to another. However, because Arvados needs specific links in place to be able to recognize the Docker image, this is a little tricky for users to do themselves.

Make sure arv-copy is usable for this task. That means:

• If you point arv-copy at an arbitrary collection, it checks for any associated Docker image links that it should copy over along with it.
• It copies over the relevant links correctly. As of b5cd9c9, I know that it creates the docker_image_hash link using the collection portable_data_hash as the name, rather than Docker's native image ID.
• Make sure this use case is appropriately documented.