Specify location of ~/.docker/config.json

[pcj]

The docker CLI has a --config flag that allows one to specify an alternate location for the docker config directory. I am primarily interested in implementing push/pull basic auth support in rules_docker https://github.com/bazelbuild/rules_docker/issues/526 and would benefit from such functionality here.

As the pusher.par already has a --config flag for a different purpose, I am proposing a flag named --client_config or similar to configure the directory where the config.json file is. This new feature would largely affect https://github.com/google/containerregistry/blob/master/client/docker_creds_.py#L222-L228.

Any objections to this? Working on a PR now.