Feature Request: GitHub Action to make flatpak alongside SNAP and WSL2

[Saijin-Naib]

I think it could be very helpful with the ease of installation for ODM to have a flatpak built alongside our SNAP and WSL2 images.

This would help greatly for platforms that lack snapd/systemd (such as Alpine, Void, etc) but which do support flatpak easily.

It would also help capture those who have an objection to using systemd/snaps for whatever reason.

Flatpak build documentation:
https://docs.flatpak.org/en/latest/first-build.html

Flatpak Github Action template:
https://github.com/marketplace/actions/flatpak-builder

I've not made any headway on this yet.