How to debug flatpak without creating new user data?

As https://github.com/orgs/PrismLauncher/discussions/2550#discussion-6847972 explains, I'm trying to debug https://discussion.fedoraproject.org/t/random-indefinite-system-hang-and-crash/110020/1#related-issues-3 using flatpak run --command=sh --devel org.prismlauncher.PrismLauncher and gdb /app/bin/prismrun. However, so that I can ensure that its environment matches mine, and so that I don't have to do an absurd amount of configuration each time I try to debug, I'd like to not have it create an entirely new userdata storage each time I debug.