This morning I've started playing with Docker, so installed boot2docker on my Mac.
All seemed to be going well, following the Working with Docker Images documentation until I tried to create a Dockerfile to install the ruby gems.
I spent a long time banging my head against the wall as to why it couldn't connect to install the gems... then I eventually worked it out, so adding this here in case anyone else has this problem in future, as there wasn't anything on Google about it.
Turns out that, because my internet bombed out during the apt-get update command, it didn't complete the update, however running it again, docker was using the cached (incomplete) data, and so if didn't work.
The fix... Run the docker build without the cache using the command...
docker build -no-cache --tag="<vendor>/<build>:<tag>" .