Arvados

This behavior should work identically to specifying a collection's portable data hash: if the user can read the collection, and it contains a single file that looks like a Docker image, record the Docker image's content address as the job's docker_image_locator.

Update the Collection.find_all_for_docker_image method to recognize and accept collection UUIDs, just like it recognizes collection PDHs. Apply the same conditions to accepting those collections: look at the manifest text, and confirm it contains a single .tar file that looks like a Docker image.

Update the docker_image documentation in /doc/user/topic/arv-docker.html and /doc/api/schema/Job.html to note that collection UUIDs are accepted.