Thank you for the quick reply! Good point about the heroku builder, I’ve removed that.
And I now understand what’s going on. You’re right, it’s a port issue. In my local .env, the PORT environment variable is set to Strapi’s usual 1337. But of course, the .env isn’t committed. In the fly.toml, I’m setting the same env var to 8080. But the Dockerfile is set to EXPORT 1337, and so Strapi is listening on a port that Docker isn’t exporting.
Once I changed PORT to 1337 too, it worked! Thanks very much for your timely help.