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).
another thing to watch out for: downstream packages (the one you have installed) are out-of-date. The current version of Docker is 0.10.0, and ships a ton of improvements. For that reason alone you should consider installing from upstream repos, at least until downstream catches up. Just keep it in a separate file in /etc/apt/sources.list.d to easily remove it later.
FYI, the "docker.io" repository in trusty points to an older version of docker (0.9.1). I believe the script at http://get.docker.io/ubuntu will install the latest version of docker.
