Add Docker Hub login to fix keychain authentication issue

Author: gyujh630

.github/workflows/ci.yml

@@ -55,7 +55,12 @@ jobs:
       run: |
         echo "${{ secrets.PROD_CONFIG }}" > .env.local
         
-    # 8. Docker 이미지 빌드
+    # 8. Docker Hub 로그인
+    - name: Docker Hub Login
+      run: |
+        echo "${{ secrets.DOCKER_PASSWORD }}" | docker login -u "${{ secrets.DOCKER_USERNAME }}" --password-stdin
+        
+    # 9. Docker 이미지 빌드
     - name: Build Docker image
       id: build
       run: |