For the use case of building pure Debian packages, we only need the
package-source-builder. So, lets just disable the rest of the images.

And also add the Debian specific bits to get the debootstrap work.

This is the same approach done in c7faa400 (Bookworm) and d285422a (Bullseye.)

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>