Bring back customizing the system config to include in the tarball

This was previously implemented in nix-community#260, but lost in the migration off
the installer tarball.

modules/build-tarball.nix

@@ -1,7 +1,7 @@
 { config, pkgs, lib, ... }:
 with builtins; with lib;
 let
-  cfg = config.wsl;
+  cfg = config.wsl.tarball;
 
   defaultConfig = pkgs.writeText "default-configuration.nix" ''
     # Edit this configuration file to define what should be installed on
@@ -21,7 +21,7 @@ let
 
       wsl.enable = true;
       wsl.defaultUser = "nixos";
-      ${lib.optionalString (!cfg.nativeSystemd) "wsl.nativeSystemd = false;"}
+      ${lib.optionalString (!config.wsl.nativeSystemd) "wsl.nativeSystemd = false;"}
 
       # This value determines the NixOS release from which the default
       # settings for stateful data, like file locations and database versions
@@ -34,13 +34,22 @@ let
   '';
 in
 {
+  options.wsl.tarball = {
+    configPath = mkOption {
+      type = types.nullOr types.path;
+      default = null;
+      description = "Path to system configuration which is copied into the tarball";
+    };
+  };
+
   # These options make no sense without the wsl-distro module anyway
-  config = mkIf cfg.enable {
+  config = mkIf config.wsl.enable {
     system.build.tarballBuilder = pkgs.writeShellApplication {
       name = "nixos-wsl-tarball-builder";
 
       runtimeInputs = [
         pkgs.coreutils
+        pkgs.findutils
         pkgs.gnutar
         pkgs.nixos-install-tools
         config.nix.package
@@ -71,7 +80,13 @@ in
         nixos-enter --root "$root" --command 'nix-channel --add https://github.com/nix-community/NixOS-WSL/archive/refs/heads/main.tar.gz nixos-wsl'
 
         echo "[NixOS-WSL] Adding default config..."
-        install -Dm644 ${defaultConfig} "$root/etc/nixos/configuration.nix"
+        ${if cfg.configPath == null then ''
+          install -Dm644 ${defaultConfig} "$root/etc/nixos/configuration.nix"
+        '' else ''
+          mkdir -p "$root/etc/nixos"
+          cp -R ${lib.cleanSource cfg.configPath}/. "$root/etc/nixos"
+          chmod -R u+w "$root/etc/nixos"
+        ''}
 
         echo "[NixOS-WSL] Compressing..."
         tar -C "$root" \

modules/default.nix

@@ -14,7 +14,5 @@
       "Additional workarounds are no longer required for Docker to work. Please use the standard `virtualisation.docker` NixOS options.")
     (lib.mkRemovedOptionModule [ "wsl" "interop" "preserveArgvZero" ]
       "wsl.interop.preserveArgvZero is now always enabled, as used by modern WSL versions.")
-    (lib.mkRemovedOptionModule [ "wsl" "tarball" ]
-      "The tarball is now always generated with a default configuration.nix.")
   ];
 }