Rework image generation to be part of main production CentOS composes

See discussion in https://github.com/centos-workstation/main/pull/48

TL;DR using the compose repos for our main builds means we get content that may not be on the mirror network yet, which can cause skew.

Building containers in compose

However, bigger picture what we want here is to always build a centos-bootc container image on every compose.

In the short term we will not run any "heavy" integration testing as part of that "compose".

The design proposed here is that pungi will call out to this repository to do a build. How that works is basically what I outlined here (in a similar Fedora discussion):

fedora/bootc/base-images!148 (comment 2417478978)

Effectively pungi is responsible for generating .repo files which it passes to this build.

In the short term we will dump .ociarchive files into the compose.

BTW I also think we should do the same for quay.io/centos/centos.

Shifting left testing

Our primary goal here must be to "shift left" our integration testing to merge requests. This will cover 95% of problems. There is a remaining 5% where two packages build and pass the bootc tests independently, but when landing together break things, but it's ok to catch that post merge for now.