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.
Move Docker doc under /user/workers

Also add the definite article.
Tweak verbiage
Eschew use of "Docker" in favor of "container"
Standardize on "container-based" instead of "Docker"
Tweak Docker doc
Add the definite articles, definitely
Tweak wording a bit
Remove mention of 'sudo' from Firefox

This anticipates the merge of https://github.com/travis-ci/travis-build/pull/325
Update docker doc location
Add documents on other VM backends
Prefer "standard" to "legacy"
Clarify caching on Docker workers
Update docker docs
Highlight virtualization env differences in the higher level doc
Change issue tracker URL

To show what's already labeled with 'docker'
Correct file system type on docker images
Merge pull request #140 from travis-ci/ha-docker-documentation

Add Docker documentation