FYI One thing that I’ve initially missed looking to optimize GH deployments is that fly deploy has the --remote-only option that allows to build image on Fly machine with dedicated CPU and persistent volume that caches Docker layers, more info here: New & Improved Remote Builders.
Edit: Along with --detach, fly.toml update (99% cached) went from ~10m to 40s! 
OMG why isn’t this marvel exposed better? 