Ah, well I assume Fly have changed how their launch process works since I last used it. That’s fine, you already have a Dockerfile and .dockerignore then. Which suggests it’s not using the alternative (the buildpack).
Since you mention Debian bullseye … I wonder if the fix is as simple as changing the quotes around your URL 
Only this person found that was the problem when they too could not connect to Planetscale (after experimenting with CA):