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.
In the Mongo tutorial it was made very clear that you can just change a few things in your current configuration, or for convenience download this ready-made one.

This is not just "new", and there should be an explanation here at least of what is different.