Docker images are pushed to ghcr.io on every commit. Try the following:
docker run --rm -it ghcr.io/supabase/nix-postgres-15:latest
Every Docker image that is built on every push is given a tag that exactly corresponds to a Git commit in the repository — for example commit d3e0c39d34e1bb4d37e058175a7bc376620f6868 in this repository has a tag in the container registry which can be used to pull exactly that version.
This just starts the server. Client container images are not provided; you can
use nix run for that, as outlined here.