ci: build and publish the docker image on release/* tags

Mirrors the reimpasto blueprint pipeline: tag release/x.y.z builds the
image (backend + WASM frontend in one multi-stage build) and pushes
gitea.woggioni.net/woggioni-opencode-agent/scopa:x.y.z plus :latest.

The build context is the repository root and the Dockerfile is
server/Dockerfile (monorepo layout). Requires the repo secret
PUBLISHER_TOKEN with write:package scope.

.gitea/workflows/build.yaml

@@ -0,0 +1,39 @@
+name: CI
+on:
+  push:
+    tags:
+      - 'release/*'
+
+jobs:
+  build_and_push_docker_image:
+    name: "Build and push docker image"
+    runs-on: hostinger
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v6
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      - name: Login to Gitea container registry
+        uses: docker/login-action@v3
+        with:
+          registry: gitea.woggioni.net
+          username: woggioni-opencode-agent
+          password: ${{ secrets.PUBLISHER_TOKEN }}
+      - name: Extract metadata for image tags
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: gitea.woggioni.net/woggioni-opencode-agent/scopa
+          flavor: latest=false
+          tags: |
+            type=match,pattern=release/(.*),group=1
+            type=raw,value=latest
+      - name: Build and push docker image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: server/Dockerfile
+          platforms: linux/amd64
+          push: true
+          pull: true
+          tags: ${{ steps.meta.outputs.tags }}