For the sake of others 
MySQL is not part of Ghost’s docker image, so you need to install it separately, when doing that you realize the real issue: MySQl8 needs a lot more RAM than fly.io offers and what most hosting platforms offer. Bare minimum one seemingly needs 2GB RAM
Conclusion: Hosting Ghost on fly.io’s (free tier) is no longer an option (PlanetScale is NOT an option for Ghost)