First local run: add supabase to instructions & improve shebang

What I had to change in order to run locally: - start supabase (was not part of CONTRIBUTING.md instructions) - modify supabase.sh to - initialize `.env.local` - use a [more generic shebang](https://stackoverflow.com/a/16365367) that supports OSes where bash is not in the default location (NixOS in my case)