PSA to anyone trying this on Ubuntu 14: the docker command seems to be "docker.io" now.
In the meantime, you should either a) install the latest docker from the official docker APT repos http://docs.docker.io/installation/ or b) do something like ln /usr/bin/docker{.io,}
In any case, don't leave your Docker install without a docker binary in your path. That will universally break all tutorials and scripts ever written for Docker, and is unsupported by us (Docker maintainers).