Add Docker documentation

This commit adds a new page on Docker workers, and adds notes to
various places where use of `sudo` is suggested.

A new CSS class is introduced to draw attention to 'sudo' issue,
but that is, of course, subject to change.

![travis_ci__the_build_environment](https://cloud.githubusercontent.com/assets/25666/4882403/c249f8d4-6355-11e4-9f72-5126b0ebe06b.png)

> allow the use of caches for free builds on travis-ci.org

?