Bug 1150921 - Problem with pushing images to public repo caused by missing .dockercfg file in $HOME.

It appears that recently `fedora:20` image changed it's `$HOME` setting. Since we plan to move to `centos7` image where also `$HOME` is set to `/root` I've decided to go this way instead of hardcoding the location. This way we'll avoid changing the location on every image/`$HOME` variable change. I'm open to discussion, though. 
This change is another requirement for #143.
Maybe we could leverage the Andy's/Caesar's `tar` streamer to copy the file into the $HOME directly, instead of bind mounting. /me just brainstorming