Little cleanup

[gw0]

This PR builds upon https://github.com/rpardini/docker-registry-proxy/pull/78 and cleans up a few things.

It contains one non-backward compatible change. It affects only those who override REGISTRIES env var, use DockerHub, but do not explicitly set REGISTRIES="auth.docker.io registry-1.docker.io". As part of the organization all default configurations were put in Dockerfile (including previously internally hard-coded values, such as DockerHub servers). This change is necessary to cover the use cases where one does not want to cache DockerHub, but wants to cache another registry. If one does not set the REGISTRIES env var (and the default in Dockerfile is used), everything works as it used to (it works even for GitLab.com).