Cannot connect to RabbitMQ via guest/guest when using the 3.9-rc image with default args

[HoloRin]

After starting Rabbit with docker run -it --rm --name rabbitmq -p 5672:5672 -p 15672:15672 rabbitmq:3.9-rc, a client on the host machine can no longer connect via the default credentials. This has worked in the past.

[HoloRin]

This is due to the removal of many configuration defaults in the new image via the changes to docker-entrypoint.sh (#424).

In particular, by default the guest user is a loopback user, which means it can only connect via localhost. In the past the docker-entrypoint.sh would remove this restriction.

Clients on the host can connect if RabbitMQ is started this way:

docker run -it --rm --name rabbitmq -p 5672:5672 -p 15672:15672 -e RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS='-rabbit loopback_users "none"' rabbitmq:3.9-rc

[HoloRin]

The way I would like to fix this is to provide a default rabbitmq.conf file in the image that covers this, which can easily be overridden with a volume if desired.