docker: security improvements

• SECURITY Makes root FS of all docker containers except of MySQL read-only. The state is now fully encapsulated in volumes.

• BETTER Uses RabbitMQ instead of Redis as a message queue. This improves permission handling and enables fine-grained control over the queue.

• Makes small improvements to the docker documentation.

Signed-off-by: Marco Neumann <marco@crepererum.net>