My trust in Flatpak diminished after installing the book reader Calibre and finding that despite the sandboxing Calibre was given blanket access to my drive. Apparently a quirk of the developer behind Calibre insisting upon it. No warnings or communication of the exception were given. All trust I had in Flatpak was eroded from that moment on. Curious about the podman options or similar. Having desktop apps in a container with selective access to system resources seems like it would be more secure and configurable if configured correctly. Flatpak as it stands seems to be a legacy solution to what should be a container and namespacing solution.
That’s what the kasmweb images on dockerhub are - desktop apps containerised
Don't know how you installed it, but required permissions are usually shown and warned about. The CLI could be clearer, though, but shows that host filesystem access is granted to calibre when it prompts to install