Build and containerize the nix-shell at build time

[haslersn]

Hi! I was looking at this template and it seems that it installs Nix at build-time and then builds & enters the nix-shell at runtime.

However, it would be nicer if the nix-shell could be built and containerized at build time. Is this possible?

In that case, the container image wouldn't even need to have Nix installed, because it's not required at runtime. This is just like a nix-shell with --pure where Nix is not installed by default. Users who want to have Nix in their development environment can add nix to the nativeBuildInputs in shell.nix.