Update Docker instructions for 0.5.3 and avoid `git clone`

Since 0.5.3 Docker listens on a socket which is only readable/writeable by `root`, all `docker` commands need to be run as a superuser.

It is also possible to build from GitHub without cloning so this pull includes that change as well.

Tested and works on Ubuntu 13.04 with lxc-docker 0.5.3 from official Launchpad PPA.