Update docker tutorial

 * Use the standard installation instruction instead of the master
   version: 0.7 has been out for some time now.
 * As a result, switch to `sudo docker` instead of `sudo ./docker`.
 * Publish ports when running buildbot to be able to log into ssh and
   see the buildbot frontend.
 * Reformat the text
However, it publishes port 22 on... port 22, so that's not a good idea if you already have a ssh server running.