Docker issues

[neworderofjamie]

The new Docker image is pretty nice but has a couple of issues:

• I didn't quite understand how layering works when I built it so the Dockerfile could be significantly optimised by combining together multiple bash incantations into single RUN commands (specifically all the apt-get stuff and the PyGenn install).
• The Python environment is currently owned by root so not modifiable from within the docker environment. It would probably be better to create a virtualenv in the Dockerfile and, in the entrypoint, move it into /home/genn, chown it to the genn user and activate it.

[neworderofjamie]

You can pip install --user whatever which I think is fine - no need mess with this any more