Dockerfile uses the existing repository, will not overwrite config.yml

With this change, I can...

- control which version of the registry I build (it's the version where the Dockerfile is located)
- control the registry configuration that will be used, because I can create a config.yml file without it being overwritten by the sample one.

... without modifying the Dockerfile.

The downside is that the image cache is blown by that new "ADD" step. The "RUN git clone" that it replaces doesn't play well with the cache, either.