PR #1789 improved various aspects of the ostree_container phase
regarding subvariant handling and filenames, this is mainly to
help us with how we want to handle bootc images (currently in
the IoT compose, but the generic base bootc image may move to
the Fedora compose).
PR #1790 rejigs the compose phase handling so the main image
build phase is not unnecessarily blocked on the ostree_install
phase. This should cut 60-90 minutes out of the main Fedora
compose time.
(cherry picked from commit 13884fef2c199b613442af03c24b737a7e3cb057)
