Provide custom toolbox images

Quoting the first commit (after the commits from !2712 (merged)):

ci: Produce toolbox images

Toolbox is a convenient option for development, but setting up
the image with all dependencies is annoying at best, in particular
later in the cycle when `dnf builddep` is likely insufficient.

To address that, produce toolbox images for main and stable branches
that are based on the regular CI image, and update them whenever
the image version is updated. This guarantees that all build- and
runtime dependencies are included.

Unsurprisingly, the script that produces the image draws heavily
from freedesktop's ci-templates. The most notable difference
(other than being neither distro-agnostic nor generic) is that
tag names are fixed (toolbox:main, toolbox:43 etc.) to make them
easier to consume.

Instead, whether an image needs rebuilding is based on a custom
label that records the base image that was used.

That's the important bit, the follow-up commits then just add a create-toolbox script to set up a toolbox with those images, and optionally performs a couple more setup tasks (like generating a gnome-builder configuration)