ci: new workflow

nixos/module/hectic/service/gitea-runner-controller.nix

@@ -54,7 +54,9 @@ let
       "GCR_ACT_RUNNER_SHA256=${cfg.actRunnerSha256}"
     ]
     ++ lib.optionals (cfg.imageId != null) [ "GCR_IMAGE_ID=${cfg.imageId}" ]
-    ++ lib.optionals (cfg.armImageId != null) [ "GCR_ARM_IMAGE_ID=${cfg.armImageId}" ];
+    ++ lib.optionals (cfg.armImageId != null) [ "GCR_ARM_IMAGE_ID=${cfg.armImageId}" ]
+    ++ lib.optionals (cfg.nixImageId != null) [ "GCR_NIX_IMAGE_ID=${cfg.nixImageId}" ]
+    ++ lib.optionals (cfg.armNixImageId != null) [ "GCR_ARM_NIX_IMAGE_ID=${cfg.armNixImageId}" ];
 in
 {
   options = {
@@ -129,6 +131,18 @@ in
           whose fallback chain includes ARM server types.
         '';
       };
+      nixImageId = lib.mkOption {
+        type = with lib.types; nullOr str;
+        default = null;
+        example = "161547269";
+        description = "Hetzner Ubuntu image id used by Nix-capable x86 runners.";
+      };
+      armNixImageId = lib.mkOption {
+        type = with lib.types; nullOr str;
+        default = null;
+        example = "161547270";
+        description = "Hetzner Ubuntu image id used by Nix-capable ARM runners.";
+      };
       actRunnerVersion = lib.mkOption {
         type = lib.types.str;
         default = "1.0.6";

nixos/system/hectic-lab/hectic-lab.nix

@@ -111,6 +111,8 @@ in {
       enable  = true;
       imageId = "424558114"; # MicroOS x86 + Hetzner datasource dhcpcd fix
       armImageId = "423979717"; # OpenSUSE MicroOS ARM K3S 2026-08-24 snapshot
+      nixImageId = "161547269"; # Ubuntu 24.04 x86; Nix needs writable root
+      armNixImageId = "161547270"; # Ubuntu 24.04 ARM; Nix needs writable root
       allowedRepos = [
         "hinterland/*"
         "yukkop/*"