This patch builds off of Martin's ansible tasks commit (rLTRN2b720ca61cb8).
I've added a Dockerfile in the 'libtaskotron/ext/' directory as
well as a module to 'libtaskotron/ext/disposable' for dealing with
Docker containers.

This expects a docker daemon to be running on the host (so is akin
to --libvirt), and will build the 'taskotron-worker' image if it's
not already found on the system.

Testing this is very similar to testing the patch Martin wrote,
except you use the '--docker' flag during invocation.

To test:
$ git clone https://github.com/stefwalter/gzip-dist-git
$ sudo runtask -d -i gzip -t koji_build -a x86_64 --docker gzip-dist-git/tests/test_rpm.yml

Read the whole conversation at https://fedorapeople.org/groups/qa/phabarchive/differentials/phab.qa.fedoraproject.org/D1196.html