add action to build demo images

.forgejo/workflows/build-images.yaml

@@ -0,0 +1,20 @@
+name: "build demo images"
+on:
+  push:
+    branches:
+      - main
+jobs:
+  build:
+    name: Build NixOS images
+    runs-on: debian-latest
+    steps:
+      - name: install sudo
+        run: apt-get update && apt-get -y install sudo
+      - name: checkout repository
+        uses: actions/checkout@v4
+      - name: install nix
+        uses: https://git.distrilab.fr/nix-clic/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-24.05
+      - name: run generator
+        run: nix-shell -p nixos-generators --run "nixos-generate -c inventory/demo-configuration.nix -f proxmox-lxc"