Validate registry publish secret

.gitea/workflows/build.yml

@@ -39,6 +39,15 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
+      - name: Validate registry token secret
+        env:
+          REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
+        run: |
+          if [ -z "$REGISTRY_TOKEN" ]; then
+            echo "::error::Set the repository Actions secret REGISTRY_TOKEN to a Gitea token with package write access."
+            exit 1
+          fi
+
       - name: Login to Gitea registry
         uses: docker/login-action@v3
         with: