add oldstable, testing and unstable images for other images

This is particularly important for golang and python, where the versions of those packages matter a lot.

For Python, in particular, we want the fabric-tasks test suite to run across a broader range of Python releases than "just stable", but this is also critical for golang, which has a tendency of upgrading itself spontaneously if you don't have a recent enough version.

For podman and redis, it seems less relevant, if not absolutely useless and not something we want. But it seems way more complicated to split those out in a different job than just doing the multiple builds for every image, so for now we're proposing to do just that.

This should also make it easier to bootstrap the podman image based on trixie when we need to, as the image will already have been built.

Alternatively, we could move most of the build-others job code into a "dot job" (like .build_debian_image) and reuse it in two different jobs.

Closes: #17