login command skipped in registry provider

[jschneiderhan]

I may be wrong here, but I think https://github.com/bflad/chef-docker/blob/master/providers/registry.rb#L29 is always returning true, causing the docker login command to always be skipped. Would checking new_resource.update_by_last_action? have the desired effect? I'm guessing the point of the check is to make sure the login command is only called once for a particular server. I'll try it out and post a PR if it works.