ci: add release workflow to build and push Docker image on tag

Triggers on v* tags, builds multi-arch images (amd64+arm64),
pushes to codeberg.org/towerops-agent/towerops-agent with
:latest and :<version> tags.

.forgejo/workflows/release.yml

@@ -0,0 +1,53 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: https://code.forgejo.org/actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: https://code.forgejo.org/actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      - name: Set up QEMU
+        uses: https://github.com/docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: https://github.com/docker/setup-buildx-action@v3
+
+      - name: Log in to Codeberg Container Registry
+        uses: https://github.com/docker/login-action@v3
+        with:
+          registry: codeberg.org
+          username: ${{ secrets.CODEBERG_USER }}
+          password: ${{ secrets.CODEBERG_TOKEN }}
+
+      - name: Extract version from tag
+        id: version
+        run: echo "tag=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"
+
+      - name: Build and push Docker image
+        uses: https://github.com/docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          platforms: linux/amd64,linux/arm64
+          build-args: |
+            VERSION=${{ steps.version.outputs.tag }}
+          tags: |
+            codeberg.org/towerops-agent/towerops-agent:latest
+            codeberg.org/towerops-agent/towerops-agent:${{ steps.version.outputs.tag }}
+
+      - name: Run tests
+        run: go test -race -v -count=1 -timeout 60s ./...
+
+      - name: Vet
+        run: go vet ./...