Different default port for worked examples

[jc-harrison]

The quick-start script starts a worked examples container, with the Jupyter interface exposed on port 8888. This is the same as the default port used by Jupyter on the host machine, so if the user already has a Jupyter server running there will be a port conflict with the worked examples.

It would be better for the worked examples container to default to a different localhost port.

[greenape]

I guess ideally it'd start on a random free port and tell you which one.

[greenape]

Hmm. That turns out to be a bit tricky - docker port should give you the host port for a container port, but is too. flakey during startup to be useful here.