Oh sorry, I just looked at your fly.toml and I don’t think it’s doing what you think it should 
deploy.release_command is a command that runs before actually deploying your new app version. These are meant to be short-lived. Usually migrations.
I assume you explicitly set a command there because your Dockerfile has a different one?
You can do:
[experimental]
cmd = ["/app/bin/app_name", "start"]