Improve docker run command in README

The -P (--publish-all) is documented as

> Publish all exposed ports to random ports

So one would have to look up the port to use on the host

    $ docker port $(docker ps -q)
    5672/tcp -> 0.0.0.0:32771
    15672/tcp -> 0.0.0.0:32770

Adjust the "docker run" command to map the AMQP and HTTP port, so users
rest of the instructions are true.

[ci skip]

README.md

@@ -76,7 +76,7 @@ for an example.
 Docker images are published to [Docker Hub](https://hub.docker.com/r/cloudamqp/lavinmq).
 Fetch and run the latest version with:
 
-`docker run --rm -it -P -v /var/lib/lavinmq:/tmp/amqp cloudamqp/lavinmq`
+    docker run --rm -it -p 5672:5672 -p 15672:15672 -v /var/lib/lavinmq:/tmp/amqp cloudamqp/lavinmq
 
 You are then able to visit the management UI at [http://localhost:15672](http://localhost:15672) and
 start publishing/consuming messages to `amqp://guest:guest@localhost`.