History and retention mechanism for bb-worker container images

Follow up of Zulip thread

Retrieving an old container image version can aid server debugging
for example, in MDEV-38613, where container MSAN instrumention evolved
over time and a clang regression was suspected.

So older container snapshots can revert the state to a working builder scenario,
and debugging can start from there.

The work was started on: https://github.com/MariaDB/buildbot/pull/907
Implementation:

• GHCR is used as the backup medium where digests are tagged with an immutable tag hist_image_commitsha
• a separate scheduled workflow will implement a 6 months retention for the package versions containing the immutable tag.