ci: Tag respective containers with latest

This is a bit of a tradeoff, we don't want silent upgrades (and hence potentially silent breaks) but we also don't want to forget to ever update things. The infra container is a particularly important one here, ideally reboots will move to the latest and greatest, and that's most convenient with a latest tag.

I added the same logic to the machine_registration container while I was at it. Since after scripts are not working, you can't do this trick with the ci-templates containers (the triggers), so for now they remain as manual tags. Perhaps another useful thing to add to ci-templates!