I think I am interested in this? I run a bunch of small web apps, currently as fly.io machines. I love fly, but it adds up when I have a bunch of small things that I want isolated — I wish I could have even smaller Fly instances. Exe.dev seems like a good middleground where I can allocate the compute from tiny to large. (?)
I use both fly and exe. Exe isn't really "docker image as the app"-focused like fly, but if you want to sort of mimic the fly deploy process you kinda sorta could make it work for you I would think. This might help:
https://exe.dev/docs/customization