Similarly to --remote or --vm plugins this plugin allows one to run the
job inside a docker container by specifying the docker image. It
executes a new container, then attaches it and uses it similarly as
--remote plugin uses remote machine.

To check it out you can use "ldoktor/fedora-avocado" image which is
available on (the default) hub.docker.com
Signed-off-by: NCleber Rosa <crosa@redhat.com>
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>