POC: use bootc for isos and image-based updates

[nikodunk]

Upside is more of the work gets moved onto podman and image-builder. Downside is it's not 100% ubuntu/debian ready yet IMO.

The flow becomes in pseudocode:

• Build: podman build Containerfile Create image. Build shenanigans are moved to Containerfile
• Install: podman run ghcr.io/osbuild/image-builder-cli:latest --type bootc-generic-iso Build ISO from image. Installer would need to run bootc install to-disk - big change I know.
• Update: podman push ghcr.io/elementary/os:latest Push image to registry so all existing ElementaryOS installs can sudo bootc update (settings app would need to be adapted), even over versions 8=>9=>10 just like Bazzite upgrades between Fedora major versions

Downsides are:

• We're relying on a not-yet stable source for our ubuntu-bootc images https://github.com/jmarrero/ubuntu-bootc. Maybe bootcrew will maintain these mid-term - tbd.
• We're relying on still-implemented composefs backend.
• Installer would need to be rewritten/wired around deploying an image with bootc install to-disk
• no sudo apt install outside of distrobox or homebrew. system becomes macos style.

Upsides:

• no hard-to-debug breakage through sudo apt install and ppa package drift
• upgrades between major elementary os releases, :latest - forever

References:

• https://github.com/jmarrero/ubuntu-bootc/
• https://github.com/bootcrew/mono
• https://github.com/ublue-os/bazzite
• https://github.com/tulilirockz/zirconium (uses mkosi/sysupdate)
• https://github.com/atomic-shindig/ubuntu-atomic-remix (uses mkosi/sysupdate)

Builds image-mode elementary OS for image-based updates and isos with CNCF bootc technology. Addresses #724 et al

[nikodunk]

So I posted this PR into the https://github.com/bootcrew discord, and @jumpyvi landed a sysupdate banger that's way more mature than this, as they noted in #793