Allow specifing docker registry credentials - card #338

This is initial commit with docker registry credentials. I'd like to hear comments whether it's better this way or to create a separate object that holds all this information? Obviously, it's only temporary solution for demo etc, and will be replaced by more generic one. Still extensive readme ahead of me.
@mfojtik @smarterclayton @csrwng 
One potential issue is that Docker creates the source on the host as a directory if it doesn't exist. `.dockercfg` is always supposed to be a file, so if it doesn't currently exist on the host, doing a build like this will result in `.dockercfg` being created as a directory, which is problematic. We probably should stat the host file and only do the mount if it exists.