Running `apt-get clean` on the latest Ubuntu and Debian images is superfluous, see also https://github.com/dotcloud/docker/blob/master/contrib/mkimage-debootstrap.sh#L157, which runs effectively `apt-get clean` after every `apt-get update`, `apt-get install`, and even disables other binary cache items just to make sure our images stay unbloated even after multiple invocations of `apt-get` inside them.