Add Gitea Actions CI to build the backend image

Builds and pushes the backend container image to the Gitea registry on git.jpaul.io on push to main and version tags, so servers pull to deploy (no build on the host). Registry credentials come from repo secrets (REGISTRY_USERNAME/REGISTRY_PASSWORD); runner label may need adjusting to the configured Gitea runner.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

.gitea/workflows/build.yml

@@ -0,0 +1,40 @@
+name: build-images
+
+# Gitea Actions build container images and push to the Gitea registry on
+# git.jpaul.io. Servers pull to deploy — no build on the host.
+#
+# Requires repo/org secrets: REGISTRY_USERNAME, REGISTRY_PASSWORD (a token with
+# package:write). Adjust the runner label to one your Gitea runner advertises.
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+
+env:
+  REGISTRY: git.jpaul.io
+  IMAGE_BASE: git.jpaul.io/${{ github.repository }}
+
+jobs:
+  backend:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to the Gitea registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+
+      - name: Build and push backend image
+        uses: docker/build-push-action@v6
+        with:
+          context: ./backend
+          push: true
+          tags: |
+            ${{ env.IMAGE_BASE }}/backend:latest
+            ${{ env.IMAGE_BASE }}/backend:${{ github.sha }}