Private Repository Support

Added:

  - Ability to specify both Authenticated and Unauthenticated private Docker repository
  - fix if container doesn't have HTTPS support in apt
  - Added custom image tags if user doesn't want to use git hashes for tagging
  - Ability to *not* overrride current `latest` tag in the repo if so desired
  - Test cases for new features
Yep, agreed. I sort of thought the same thing as I was writing it.

Would expected behaviour to assume index.docker.io is the endpoint if `registry_login_url` isn't specified?