perf(ci): Skip Docker Verify on main push - PR only

CI 優化: Docker Verify 改為只在 PR 時執行
- main push 跳過 (CD 會構建)
- 預估省下 10-15 分鐘

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/ci.yaml

@@ -235,10 +235,12 @@ jobs:
           spectral lint docs/api/api-contract.yaml || true
 
   # ==================== Docker Build Verify ====================
+  # 🚀 優化 (2026-03-24): 只在 PR 時執行，main push 跳過 (CD 會構建)
   docker-build:
     name: Docker Verify
     runs-on: [self-hosted, harbor, k8s]
     needs: [test, api-test, build]
+    if: github.event_name == 'pull_request'
     timeout-minutes: 20
     strategy:
       matrix: