Update entrypoint.sh

[wbg]

added $AIRFLOW__CELERY__NO_DEFAULT_BROKER_URL to allow for alternative brokers (rabbitmq in my case) using airflow.cfg

[NBardelot]

Hi,

The AIRFLOW__CELERY__NO_DEFAULT_BROKER_URL implies that there is an existing Airflow configuration in airflow.cfg in the [celery] section, named no_default_broker_url. That does not exist.

What you want it to explicitly provide the AIRFLOW__CELERY__BROKER_URL with your own broker's URL. But currently the entrypoint's behaviour is to override it by computing a new value using REDIS_* variables.

That's one of the things I want to fix in a PR I've just submitted. Please put a comment in my PR if you're interested:

#495