add artifact upload step in build image action

.forgejo/workflows/build-images.yaml

@@ -4,6 +4,13 @@ on:
 #  push:
 #    branches:
 #      - release
+
+# Allow only one concurrent image build, skipping runs queued between the run in-progress and latest queued.
+# However, do NOT cancel in-progress build as we want to allow this to complete.
+concurrency:
+  group: images
+  cancel-in-progress: false
+
 jobs:
   build:
     name: Build NixOS images
@@ -14,4 +21,12 @@ jobs:
       - name: checkout repository
         uses: actions/checkout@v4
       - name: run generator
-        run: nix-shell -p nixos-generators --run "nixos-generate -c inventory/demo-configuration.nix -f proxmox-lxc"
+        run: nix-shell -p nixos-generators --run "nixos-generate -c inventory/demo-configuration.nix -f proxmox-lxc -o nixin-image-proxmox-lxc"
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: nixin-proxmox-lxc-template
+          path: ./nixin-image-proxmox-lxc/tarball/nixos-system-x86_64-linux.tar.xz
+          if-no-files-found: error
+          compression-level: 0
+          overwrite: false