This is on the roadmap. The open question right now is if we can just do "fork from checkpoint" for customized template environments, or if we need all the docker infrastructure.

If the fat bundled environment harmful for you, or just extra stuff you don't care about?

Not harmful for now - "fork from checkpoint" would be perfectly fine for me at the moment. The main issue (as flagged in the post) is that setting up additional tooling can take a while!

In the longer term, docker is nice from a reproducibility + CI perspective, and a docker build is already something can easily work with and track in my system.

One thing I've heard but not verified with other sandboxed execution providers is that startup times for custom images can be quite slow, so it could be a potential differentiator given Fly's existing infra.