Prebuilt docker will start from directory /notebooks

[janpfeifer]

In order to hide the default user home directory, now the Docker will start from /notebooks, and the instructions suggest mounting the users current directory in a sub-directory called hosts/.

If one is using (by copying from the Docker container) the previous directory /home/jovyan, this may break them.

This is unlikely, but just a heads-up next time one pulls a new image.