There is an ongoing conundrum in the downstream Debian/Ubuntu packaging of Docker. Our friends at Debian and Ubuntu are doing their best to solve it.

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).