RABBITMQ_CONFIG_FILE is set to "/var/lib/rabbitmq/rabbitmq.conf" but docker-entrypoint.sh waits it to be without ".conf" (https://github.com/docker-library/rabbitmq/blob/master/3.7/debian/docker-entrypoint.sh#L201-L203)
So in your version it creates new file rabbitmq.conf.conf and after new params are written there (i.e. RABBITMQ_DEFAULT_USER and RABBITMQ_DEFAULT_PASS - rabbitmq-server process doesn't see these params)
RABBITMQ_CONFIG_FILE is set to "/var/lib/rabbitmq/rabbitmq.conf" but docker-entrypoint.sh waits it to be without ".conf" (https://github.com/docker-library/rabbitmq/blob/master/3.7/debian/docker-entrypoint.sh#L201-L203) So in your version it creates new file rabbitmq.conf.conf and after new params are written there (i.e. RABBITMQ_DEFAULT_USER and RABBITMQ_DEFAULT_PASS - rabbitmq-server process doesn't see these params)