ci: Update to match current rpm-ostree

ci: Update to match current rpm-ostree

This copies the `ci/` directory from rpm-ostree, with much the same rationale;
among other things we don't want to depend on the Docker hub.

The specific reason I'm doing this is that I want to add a CentOS7 build, but
that means we can't use `projectatomic/ostree-tester`, and at that point we
might as well unwind it all.

Closes: #917
Approved by: jlebon