[jbergknoff]

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.