MDBF-850 - Backup PROD master tag

[RazvanLiviuVarzaru]

Main feature: MDBF-791 - Run Production BuildBot services in docker containers

Background:

• when the branch is Main, create a copy of the Production tag {previous_master, previous_master-web}.
• in the future after the backup step, a "Move Tag" step will be added. For the moment we move dev_ tag to production manually.

In this patch:

• add MAIN_BRANCH environment variable. If branch is not main, no build, no backup creation.

@fauust @vladbogo let me know if that's a suitable approach.
Do you want backup on GHCR also?

Thanks.

[fauust]

Do you want backup on GHCR also?

Yes please, should be fast and pretty straightforward.

The rest looks good