Improve container image workflows by providing a mtree.ignore and a way to monitor uploads

Description

Right now there's no way to get alerts if an image is updating a lot (say when things should be added on mtree.ignore).

A mtree.ignore with working defaults could be provided during container builds.

Also, a way to monitor images being upload too often (except of course in cases like Debian unstable) could help spotting files that should be added into mtree.ignore.

Related to onimages#6 (closed).

Tasks

• Use a default mtree.ignore which works for most scenarios (Debian and Alpine Linux at least).
• Bonus: create a procedure (or improve the container build job) to detect sequential uploads across daily runs, which can mean the repository's mtree.ignore needs update.
• Inform TPA on the improvements, so they can evaluate and eventually implement these things.