Docker registry credentials

[mprzysucha]

No description provided.

[marcus-drake]

Thanks @mprzysucha !

I have looked into this PR now. I'm questioning if this should be added to the plugin.
Would it be equivalent to run docker login -u {user} -p {password} {url} in a shell before sbt is started?
Or that it runs as a dependency task to the dockerBuildAndPush or dockerPush tasks?
Something like:

scala.sys.process.Process(Seq("docker", "login", ...)).run().exitValue()