dockerTools: Properly add /nix/ and /nix/store/ first to layer.tar

In https://github.com/NixOS/nixpkgs/pull/58431 the authors ensured that
the resulting layer.tar would always list

  /nix/
  /nix/store/

first to fully comply to the tar spec. Various refactorings later it is only
ensured to create /nix/ but NOT /nix/store anymore. Instead tar transformed
them to /nix/nix and /nix/nix/store.