If you haven’t already, see it this alternative tutorial by a community member (albeit deploying Spring with Postgres) has any hints for you: Deployment of Java Spring API using Dockerfile - #8 by renanfranca
Here’s another one (no databases that I see): Java app is killed on startup - #5 by ignoramous
Hm, a shot in the dark: Check if local-only builds / deploys work, flyctl deploy --local-only <other-args>?