Don't upgrade in builds

Updates will be baked into the based images you don't need to apt-get upgrade your containers. Because of the isolation that happens this can often fail if something is trying to modify init or make device changes inside a container. It also produces inconsistent images because you no longer have one source of truth of how your application should run and what versions of dependencies are included in the image.
source: http://crosbymichael.com/category/docker.html