The issues you ran into with older versions of Rails was probably because the Dockerfile that `fly launch` generated was for new versions of Rails. We switched to https://github.com/rubys/dockerfile-rails to streamline Dockerfile generation and support older versions of Rails.
I encountered a few issues. One was definitely something to do with older postgres. From memory, I tried downgrading it using apt, but then other things played up and I put the project aside.
I followed Sam's suggestions to regenerate the Dockerfile with dockerfile-rails (btw, thanks for your work on dockerfile-rails, super excited about it) and solved a couple more issues, but I again ran out of steam when new issues kept coming and coming. I'm sure when I'm more comfortable with docker these will become trivial to solve.
These were not super determined attempts by me, more playing around. I look forward to more serious attempts when I'm more capable with docker.
The issues you ran into with older versions of Rails was probably because the Dockerfile that `fly launch` generated was for new versions of Rails. We switched to https://github.com/rubys/dockerfile-rails to streamline Dockerfile generation and support older versions of Rails.
If you try it again and run into issues you can open an issue at https://github.com/rubys/dockerfile-rails/issues or post in https://community.fly.io and somebody will help get that sorted out.
The more versions of Rails we can deploy the better!