When I built and tested the app on Fly, I got a response from Fly member shortly after.
Therefore, I published a simple sample on the GitHub repository.
If you test this, you can keep using it for free. Fly offers a free tier of $15 per month.
The latest Fly deploys Docker Image. It works the same as Google Cloud Run.
The only difference is the port specification and fly.toml.
Comments posted in my past are valid for Fly: