docs: CI 구성에 레디스 추가

.github/workflows/backend-test-when-pr.yml

@@ -18,6 +18,18 @@ jobs:
     defaults:
       run:
         working-directory: ./
+
+    services:
+      redis:
+        image: redis
+        options: >-
+          --health-cmd "redis-cli ping"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+        ports:
+          - 6379:6379
+
     steps:
       - uses: actions/checkout@v3
       - name: Set up JDK 17