ci: add test-demo workflow

Closes: ngi-nix#698

Author: imincik

.github/workflows/test-demo.yaml

@@ -0,0 +1,56 @@
+name: Test VM demo
+
+on:
+  push: { branches: [ 'main' ] }
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: 'actions/checkout@v4'
+      - uses: DeterminateSystems/nix-installer-action@v16
+      - uses: cachix/cachix-action@v14
+        with: { name: 'ngi' }
+
+      - name: Build overview
+        run: nix build .#overview
+
+      # FIXME: download this file from overview
+      - name: Download default.nix file
+        run: >
+          cat <<EOF >default.nix
+          {
+            ngipkgs ?
+              import
+                (fetchTarball "https://github.com/eljamm/ngipkgs/tarball/init-project-demos/8eb7f038fd62fd6490e017d78e6a30e7b64a13fa")
+                { },
+          }:
+          let
+            servicePort = 9000;
+            domainName = "localhost:${toString servicePort}";
+          in
+          ngipkgs.demo {
+            services.cryptpad = {
+              enable = true;
+              settings = {
+                httpPort = servicePort;
+                httpAddress = "0.0.0.0";
+                httpUnsafeOrigin = "http://${domainName}";
+                httpSafeOrigin = "http://${domainName}";
+              };
+            };
+
+            networking.firewall.allowedTCPPorts = [ servicePort ];
+            networking.firewall.allowedUDPPorts = [ servicePort ];
+          }
+          EOF
+          
+      - name: Build VM
+        run: nix-build default.nix
+
+      - name: Run VM
+        run: ./result &
+
+      - name: Test application
+        run: curl --retry 10 --fail localhost:9000 | grep CryptPad