If you run bin/rails generate dockerfile as described in Dockerfiles and fly.toml · Fly Docs and get the error below, you may need to add dockerfiles-rails gem to your Gemfile!
hac@MBP tcard % bundle exec rails generate dockerfile
Could not find generator 'dockerfile'.
Run `bin/rails generate --help` for more options.
Once Add an aside callout about dockerfile-rails gem by heliocola · Pull Request #1184 · superfly/docs · GitHub gets merged, this How To may become obsolete!