Using buildLayeredImage to create containers, how do I get the systemd service files?

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Say that I’m building containers using a services.nix like so:

let
  config = import ./config.nix;
  pkgs = config.pkgs;
  postgres = import ./services/postgres.nix;
  searx = import ./services/searx.nix;
in rec {
  serviceimages = pkgs.writeText "images.ini" ''
    [containers]
    postgres=${postgres(pkgs)}
    searx=${searx(pkgs)}
  '';
}

with config.nix just containing the version hash (yes I know I’m out of date, I’m getting to it!)

  # nixos-23.05
  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/fcc147b1e9358a8386b2c4368bd928e1f63a7df2.tar.gz") {};

The actual service nix files look like the following:

{ pkgs, ... }:
with pkgs;
let
  entryPoint = writeShellScriptBin "entrypoint.sh" ''
    #!/bin/sh
    set -Eeuo pipefail

    PGUSER=${PGUSER:-postgres}
    PGDATA="/postgres"
    export PGDATA

    if [[ ! -d "$PGDATA" ]]; then
      mkdir -p "$PGDATA"
      initdb --auth=trust --encoding=UTF8 --no-locale -U "$PGUSER"

      cat >> "$PGDATA/postgresql.conf" <<-EOF
        listen_addresses = '*'
        unix_socket_directories = '$PGDATA'
    EOF
    fi

    exec postgres

  '';
in
  dockerTools.buildLayeredImage {
    name = "postgres";
    contents = [ busybox bash postgresql_14 entryPoint ];
    config = {
      Cmd = [ "entrypoint.sh" ];
      Env = [];
      Volumes = { "/postgres" = {}; };
    };
  }

I can use nix-build to get container images I can load into docker or podman. I’m creating my own entrypoint here to run Postgres. I was considering using superd to just run the systemd target file. However, if I look in these containers, I don’t see the .service file in the nix store or in the out directory. I’ve been looking around at documentation and it seems like the package has to be added as a systemPackage in order to get the systemd service files? But I’m not sure how that works with buildLayeredImage