Something the article doesn't touch on in its pursuit of a smaller image - when you run "apt-get install" or "apt-get upgrade" you should do "&& apt-get clean" in the same RUN command.
This will remove the .debs apt just downloaded and installed that are being cached in /var/cache/apt/archives, saving you a little disk space.
Thanks for pointing this out. In the "debian:wheezy" docker image, "apt-get clean" didn't seem to have an effect. I dug around a bit and found that, in this image, aptitude is configured to not cache the downloaded packages (via /etc/apt/apt.conf.d/docker-clean).
On one hand, I consider this a good default behavior for building docker containers, so I'm glad it's there. On the other hand, I didn't know about it until I investigated. It's strong evidence for the great point @amouat made regarding base images being blackboxes in most (all?) cases.
This will remove the .debs apt just downloaded and installed that are being cached in /var/cache/apt/archives, saving you a little disk space.