I copied the Dockerfile from https://github.com/kissgyorgy/redbean-docker/blob/master/Dockerfile-multistage
I had to run this command to create the app on Fly:
fly apps create redbean-on-fly
This requires an assets/ folder with code to deploy. I made one like this:
mkdir assets
cd assets
wget https://lite.datasette.io/index.html
wget https://lite.datasette.io/webworker.js
Then I ran this in the same directory as fly.toml:
fly deploy