Fix error message in production entrypoint.sh (#8396)
Fix error message in production entrypoint.sh (#8396)

Fix non-existent BACKEND environment variable, replace with DB_URL
wooga/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,danielvdende/incubator-airflow,DinoCow/airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,wooga/airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,sekikn/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,apache/incubator-airflow,dhuang/incubator-airflow,apache/airflow,cfei18/incubator-airflow,apache/airflow,DinoCow/airflow,dhuang/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,DinoCow/airflow,bolkedebruin/airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,apache/airflow,wooga/airflow,sekikn/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,DinoCow/airflow,wooga/airflow,apache/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,mistercrunch/airflow,mistercrunch/airflow,apache/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow