do not use docker cache when building base image

I have just encountered a problem building docker, due to something I think related to caching.
The build of base image failed during apt-get install, as it tries to use a package and gets 404-not-found.
I think it is because 'apt-get update' got cached and is not effectively executed for the base container but skipped. Probably related to "unless the RUN command *itself* changes (and thus invalidates Docker’s on-host cache), Docker will reuse the previous results from cache" in http://thenewstack.io/understanding-the-docker-cache-for-faster-builds/.

This behavior might be specific to the revision of the docker ubuntu image I have on my PC.

$ sudo docker images ubuntu | grep 14.04
ubuntu              14.04               99ec81b80c55        6 months ago        266 MB

Anyway, this patch has solved the problem for me.