add a test build for PRs

.gitea/workflows/docker-build.yml

@@ -0,0 +1,35 @@
+name: Docker Build (PR)
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+permissions:
+  contents: read
+
+jobs:
+  build-image:
+    name: Build Docker image
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@master
+        with:
+          fetch-depth: 0
+          fetch-tags: true
+
+      - name: Derive lowercase repo name
+        run: |
+          echo "REPO_NAME=$(echo '${{ github.repository }}' | tr '[:upper:]' '[:lower:]')" >> $GITHUB_ENV
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build image (no push)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          push: false
+          tags: ${{ secrets.REGISTRY }}/${{ env.REPO_NAME }}:pr-${{ github.event.number }}