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` 
:open_mouth: https://github.com/Clever/go-dockerclient/commit/08b787311bb3860a06a59ae442cb918f902b9d09