Dockercfg based registry auth

This brings parity with the behavior for the docker CLI client and allows setting authorization using configuration management systems.

Changed from using the `HOME` environment variable used by _docker CLI_ to find `.dockercfg` to `DOCKERCFG_PATH` 
Does docker actually use two formats or is it some kind of backwards compatibility guarantee?

I'm fine with supporting only the format available in most recent versions of docker.