Add support for docker hub private registry credentials

[rnorth]

Closes #819 (I wanted to rebase onto master, so thought it best to start with a fresh branch and PR).

Fixes #820

This comment describes the approach:

This is still WIP, but tackles the two underlying issues that you and others have encountered:

• basic auth credentials not being parsed out of the config.json file (for all private repos)
• registry auth for index.docker.io being rejected. To resolve this I've overridden the effectiveAuthConfig method of DockerClientConfig for now, to try and come up with a solution that doesn't require changing docker-java (yet).

In the long term, this requires a fair amount of modification to docker-java to do it cleanly, but for now we can basically layer this on top and refactor later.

[rnorth]

Thanks @bsideup, I’ll merge now.

[rnorth]

We have this out in a Release Candidate build (1.9.0-rc1) for anyone who is keen to try it!

Release notes