Update Makefile to use "docker:GIT_BRANCH" as the generated image name for increased versatility

... and other minor Makefile improvements :)

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3494 from tianon/makefile-use-branch-tag

Update Makefile to use "docker:GIT_BRANCH" as the generated image name