This introduces a Justfile and Dockerfile to enable building rpm-ostree
from source in a container, following the pattern established in the
bootc project.
See the `Justfile` for key entrypoints. Those are now used
in a new GHA flow, which we'll try to move things over to.
A key difference though vs bootc is because rpm-ostree has a lot
of C++ too we use sccache which greatly speeds things up across
incremental rebuilds.
Just one side cleanup of this is before it was *terribly*
painful and manual to hack on `test-container.sh`, and now it's easy,
fast and optimized.
Assisted-by: Claude Code (Sonnet 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>
