Describe the bug
The "Environment variables" section of the documentation reads "All Flower options must be prefixed with FLOWER_" but, in fact, the container expects the broker_url parameter to be set in the env variable CELERY_BROKER_URL.

Also, the broker_url variable is not documented, while the broker_api is, which is also misleading because setting it causes Error 500 and Connection refused errors and prevents the service to start.