Let me replay back to you what I’m reading and you can correct me if something seems off. It looks like you’re looking to boot a MySQL instance, then boot a web app that has a volume. Let’s break that up a bit.
For running a MySQL instance I’d check here Use a MySQL Database · Fly Docs that should get you rolling. Though, running a database on your own can be a bit much, so if you just need something you can boot quickly, I would also consider one of the more database-focused solutions like https://planetscale.com.
For the volume piece, you should be able to get started here: Volumes · Fly Docs