I just learned a few important things from @renchap. Apparently the docker-compose.yml in the root of the mastodon repo is not mean to be used for development. It is meant for production deployment. That's a bit confusing, but I'll accept it.
However they have made significant improvements to the dev containers implementation after I said it didn't work. It seems much more viable now.
https://github.com/mastodon/mastodon/blob/main/.devcontainer/docker-compose.yml
@polotek thanks for the writeup!
I think I'll stick with @grunfink's snac (https://codeberg.org/grunfink/snac2)? I realize there are Docker instructions available for that too, but avoiding unnecessary layers of abstraction is a bigger win for me, every time.
@renchap