ci: optimize workflow runs

- Add Docker build cache for faster builds
- Increase sync frequency from daily to every 6 hours
- keep latest 3 workflow runs only
Signed-off-by: JohnsonRan <me@ihtw.moe>

.github/workflows/sync.yml

@@ -5,7 +5,7 @@ permissions:
 
 on:
   schedule:
-    - cron: "0 4 * * *" # At 12PM UTC+8
+    - cron: "0 */6 * * *" # run every 6 hours
   workflow_dispatch:
 
 jobs:
@@ -33,4 +33,12 @@ jobs:
         if: failure()
         run: |
           echo "[Error] Due to a change in the workflow file of the upstream repository, GitHub has automatically suspended the scheduled automatic update. You need to manually sync your fork."
-          exit 1
+          exit 1
+
+      - name: Delete workflow runs
+        uses: Mattraks/delete-workflow-runs@main
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          repository: ${{ github.repository }}
+          retain_days: 0
+          keep_minimum_runs: 2