dcao-publish - support a way of tagging and pushing images

[dnephin]

There's still one piece missing from this workflow.

At the very least, there needs to be a way to "docker push" all the images that are part of the composition.

An optional extra feature would be a way to tag images from the docker-compose name of <project>_<service> to a more canonical public name. This isn't necessary if some other build tool is being used, and compose isn't responsible for building the image.