A quote from that blog post:
At Fly, you’ll have already discovered that when you create an app, it appears as http://appname.fly.dev and if you connect there, your browser switches automatically, by redirection, to https://appname.fly.dev. That’s how we do it for the fly.dev domain.
Kurt strikes again:
In my opinion this is not feasible in lots of scenarios, as it would require modifying the source code for existing distributed images.
What kind of app are you running?
You may be able to find reusable bits for doing this somewhere on this forum, or in Fly.io Apps · GitHub