Add a YAML file for the new Red Hat CI framework.
Rather than re-installing all the build deps everytime, which takes
time, I added a Dockerfile that we can wire up to the Docker Hub. For
now it lives at `jlebon/ostree-tester:rhci`, but we can move it under
the `projectatomic` org (or a new `ostree` org).
Closes: #535
Approved by: cgwalters
Jonathan Lebon