ci: Add prow/ subdirectory with Dockerfile

ci: Add prow/ subdirectory with Dockerfile

I'd like to add OpenShift's prow to this repository.  Let's start
by adding a Dockerfile - it doesn't really do anything besides build.

However...I've lately been thinking about e.g. shipping the ostree tests
as an image, and then e.g. we could test FCOS by running that container
(which would orchestrate the *host's* ostree).

Anyways, not doing that right now but this is a start.

Also this cherry picks the fix from rpm-ostree CI for the sad
Fedora release package brokenness.

Closes: #1906
Approved by: cgwalters