Postgres docker example - fixes 459

Fixes #459. Dave Gebler's work mostly, with some tweaks by me to change it to use official image.

Given tomorrow's schedule I suggest reviewer (or someone in UK, anyway) address review comments and merge, rather than waiting for me to address anything.
It's not strictly *necessary* to have completed the tutorial first (only the VM set-up portion of it), however this example doesn't go in to as much depth as the primary tutorial, hence the recommendation that people follow that first.