Hi there,
I was trying to get Phoenix + SQLite working on Fly.io and I stumbled across this gist which gave details on how to fix everything: Phoenix + SQLite Deployment tips · GitHub
It would be helpful to future users if this were either in the docs somewhere, or if the fly tool was updated to handle SQLite-based phoenix projects. I was mainly convinced to go with SQLite based on the blog post here: I'm All-In on Server-Side SQLite · Fly 
Cheers!