Downloads: add a 3.9-rc.1 Docker image one liner
Downloads: add a 3.9-rc.1 Docker image one liner

As of 3.9, the image no longer sets any defaults for remote
access of the default user. Security vs. usability at odds, again.

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website