Support running Nix as a non-root user

[exarkun]

Currently if the Docker container is started as a non-root user (eg nobody), Nix commands fail with permission errors. For example:

❯ docker run --rm -it --user nobody nixos/nix:2.16.1
bash-5.1$ nix --extra-experimental-features 'flakes nix-command' profile install nixpkgs#hello
error: could not set permissions on '/nix/var/nix/profiles/per-user' to 755: Operation not permitted

It would be nice if it were possible to use these commands as some non-root user (not necessarily "nobody") without further modifying the image.

[the-sun-will-rise-tomorrow]

❯ docker run --rm -it --user nobody nixos/nix:2.16.1

• This repository is for the nixpkgs Docker Hub organization. The nixos/nix image seems to be maintained in the nix repository, not here, so issues should probably be filed there.

• There is already an issue on this subject in this repository: #41

[the-sun-will-rise-tomorrow]

I made a PR for nixos/nix: https://github.com/NixOS/nix/pull/9854

@exarkun Maybe you can test it?