Add debian12-flatpak docker image, with the necessary tools for building a flatpak. This image also runs the dbus service so flatpak as non-root can work. Add a repackage-flatpak task for the linux64/opt build and on-change l10n, and one for linux64-shippable/opt with the full set of langpacks. Differential Revision: https://phabricator.services.mozilla.com/D238801