A few little Dockerfile tweaks

* Setting `ENV DEBIAN_FRONTEND noninteractive` is not recommended (see Note here: http://docs.docker.io/reference/builder/#env).
* Universe repository is already added in upstream image.
* A few tweaks from http://crosbymichael.com/dockerfile-best-practices-take-2.html (apt-get upgrade in upstream, grouping of install for better caching).