If you're interested in another approach to containerizing GUI applications, also checkout out x11docker. It's a small independent project maintained by one guy, nothing big like flatpak, but also pretty cool. The name is actually a bit limiting -- it supports both docker and podman, and can run wayland apps as well. One of the coolest features, in my opinion, is the ability to run a separate X server inside every sandbox and forward individual windows to the "host" X server. That way you can prevent apps from spying on your keyboard or other apps' windows.