Support alternative docker registries for root images

[thomas-riccardi]

DMakes directly calls the docker registry to get the current digest associated with a docker image+tag, assuming it's on the docker hub registry.

We should support alternative registries for root_image.

TODO

• stop hardcoding the registry url: extract it from the image:tag string somehow
• stop assuming there is only one / and only one : in the image:tag string in

  dmake/dmake/docker_registry.py

  Line 59 in 1bfcf38

  def parse_docker_image(image):

  : see https://github.com/moby/moby/blob/master/image/spec/v1.2.md and https://docs.docker.com/registry/spec/api/#overview