Arvados

Clients should be able to ask the API server for the UUID and/or content hash of a collection that a given docker image name and tag resolve to. e.g.:

arvados.api('v1').docker_images().show(name='arvados/jobs', tag='latest').execute()
arvados.api('v1').docker_images().list(name='arvados/base', tag='7.1').execute()

Motivation: a single name+tag combination may resolve to many images. The API server chooses one based on some subtle logic around the image_timestamp and created_at fields of the docker links. It's better for a client to be able to query the API server for the "current" version of an image than it is to reproduce this logic on the client side.