ci: use liskin/gh-workflow-keepalive

The objective is to maintain the CI active even after 60 days of
inactivity in the repository. This will ensure haproxy-qns image is
always up-to-date.

.github/workflows/docker_auto.yml

@@ -26,3 +26,13 @@ jobs:
         with:
           push: true
           tags: ${{ env.DOCKER_IMAGE }}:latest
+
+  workflow-keepalive:
+    if: github.event_name == 'schedule'
+    runs-on: ubuntu-latest
+    permissions:
+      actions: write
+    steps:
+      - uses: liskin/gh-workflow-keepalive@v1
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}