Fix (mostly) the DockerRegistry class which was not very useful.

The `DockerRegistry` class was not generally very useful the way it was set up. There are still shortcomings in this fix as it doesn't support registry auth, but it is more generally useful.  It now points to the main public Docker repository at the Docker Hub.

Sadly, this class is lacking in tests and needs a full suite!

@jessedearing @bryannewrelic review appreciated.