Add install nix and build container steps

.gitea/workflows/build-container.yml

@@ -1,10 +1,16 @@
-name: Gitea Actions Demo
+name: Build container
 on: [push]
 jobs:
   build-container:
-    runs-on: node-22-bookworm
+    runs-on: ubuntu-22.04
     steps:
-      - run: echo echo "🎉 The job was automatically triggered by a ${{ github.event_name }} event."
       - name: Checkout repo
         uses: actions/checkout@v4
       - run: ls -l
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+        with:
+          enable_kvm: false
+      - name: Build container
+        run: nix build .#docker
+      - run: find result/*