We need at the very least "manifest reproducibility" and so cannot just use a floating tag in our Containerfile.

Start pinning it and add a process that bumps it. There's a lot of details there to be fleshed out. See discussions in https://github.com/coreos/fedora-coreos-tracker/issues/1861.

This requires https://gitlab.com/fedora/bootc/tracker/-/issues/34.

This also relates to using `--locked` for FCOS (see https://gitlab.com/fedora/bootc/base-images/-/issues/55).