fix celery config (#22566)

Signed-off-by: kenwoodjw <blackxin55+@gmail.com>

docker/.env.example

@@ -289,6 +289,7 @@ REDIS_CLUSTERS_PASSWORD=
 # If use Redis Sentinel, format as follows: `sentinel://<sentinel_username>:<sentinel_password>@<sentinel_host>:<sentinel_port>/<redis_database>`
 # Example: sentinel://localhost:26379/1;sentinel://localhost:26380/1;sentinel://localhost:26381/1
 CELERY_BROKER_URL=redis://:difyai123456@redis:6379/1
+CELERY_BACKEND=redis
 BROKER_USE_SSL=false
 
 # If you are using Redis Sentinel for high availability, configure the following settings.