Add easy ability to publish a Docker image

[C0deH4cker]

Now that #9 has landed, it would be great if there were an easy way to publish the entire Docker image. Whether as a .tar.gz image or by pushing to the Docker Hub is still TBD.

[C0deH4cker]

I think it would be good for projects to need to opt-in to publishing by setting something like DOCKER_PUBLISH := 1 in their Build.mk file. Then, make docker-publish or make docker-push will push the publishable images to Docker Hub. That way it's hard to accidentally publish a Docker image.