This creates two images:
* ubuntu-build is a basic Ubuntu Trusty image with the build prerequisites
installed via MozBootstrap (plus some additional requirements). It also
contains the worker user and basic directory structure expected by
Mozharness.
* desktop-build is a refinement of ubuntu-build with specifics for building
Firefox Desktop (and, as it turns out, Firefox for Android). It sports a
`bin/build.sh` which acts as a fairly generic mozharness-runner that first
checks out the desired source code revisions, then invokes Mozharness. It
supports:
* caches -- tooltool, workspace, tc-vcs
* starting and stopping Xvfb if necessary (desktop tests require this)
* specifying mozharness build variant, branch, and build pool
* supplying a RelengAPI token
* copying uploads to the artifacts directory
