management image variants does not support rabbitmq plugins packaged as directories

[HoloRin]

rabbitmq/rabbitmq-server@25f45fc has changed the default for packaging rabbitmq's included plugins - they are no longer compressed files, and instead are expanded directories. This breaks the -management images who expect to perform the extraction.

See rabbitmq/rabbitmq-server#2838 for one solution.

[yosifkit]

Thanks for the heads up! Working on a PR now.