docker-library/ruby — pull request #6 by josephpage (2014-08-08 14:55:14+00:00)
Title: Cleaning up apt cache to optimize disk space

I tested it and got this :
before : 1.561 GB
after : 1.529 GB
All this 32MB is unnecessary files, as recommended in Debian mkimage :
"delete all the apt list files since they're big and get stale quickly
this forces "apt-get update" in dependent images, which is also good"
https://github.com/docker/docker/blob/master/contrib/mkimage/debootstrap