feat: build Docker variants for Debian 12 and 13

[lucasmirloup]

We currently use the default OS variant of the PHP Docker image (Debian Trixie (13), used to be Debian Bookworm (12)).

As Debian 13 removed some packages used by our projects (wkhtmltopdf for example), we need to provide a Debian 12 variant of our PHP Docker image to prevent these projects from being stuck in an outdated version.