Container immediatly exiting, when going through Manage Data/User Guide

When going through the Manage data in containers/Mount a host directory as a data volume part of the User Guide
( I had the problem, that when running the command

$ docker run -d -P --name web -v /src/webapp:/opt/webapp training/webapp python

the container would exit immediatly. While this is not exactly an error i found it a bit confusing being a complete newcomer and all. What happens is that you mount the host directory in the place where the file is stored and the container can’t execute the command when starting. Maybe a note in the doc would be helpfull.