ci: allow optional Docker Hub login for buildx pulls

.gitea/workflows/backend-docker.yml

@@ -176,6 +176,20 @@ jobs:
 
           docker buildx inspect --bootstrap
 
+      - name: Login Docker Hub (optional)
+        shell: bash
+        env:
+          DOCKERHUB_USERNAME: ${{ secrets.DOCKERHUB_USERNAME }}
+          DOCKERHUB_TOKEN: ${{ secrets.DOCKERHUB_TOKEN }}
+        run: |
+          set -euo pipefail
+
+          if [ -n "${DOCKERHUB_USERNAME:-}" ] && [ -n "${DOCKERHUB_TOKEN:-}" ]; then
+            echo "${DOCKERHUB_TOKEN}" | docker login docker.io --username "${DOCKERHUB_USERNAME}" --password-stdin
+          else
+            echo "Docker Hub credentials not configured, continuing with anonymous pulls."
+          fi
+
       - name: Build and push image
         shell: bash
         env: