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.
I don't think we need to go into that level of detail for this; it's pretty obvious and this isn't a tutorial, it's an example (so e.g. we don't say how to download postgres client).