docker: configure and document the APP evironment variable for celery
and update CELERY_BROKER_URL as BROKER_URL; CELERY_BROKER_URL seems to be still working, but the documentation now only mention the BROKER_URL variablesince, see:
https://docs.celeryproject.org/en/latest/reference/cli.html
Migrated from D6444 (view on Phabricator)