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/docker-image.yml

@@ -40,4 +40,14 @@ jobs:
           platforms: linux/amd64,linux/arm64
           tags: |
             ghcr.io/${{ github.repository_owner }}/moontv:latest
-            ghcr.io/${{ github.repository_owner }}/moontv:${{ github.sha }} 
+            ghcr.io/${{ github.repository_owner }}/moontv:${{ github.sha }} 
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - 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