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