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.
This is like a continuation of the last sentence but it isn't (because "And" is capitalised). Again it would be nice to have this wording consistent with the MongoDB tutorial - "docker ps shows that no containers are running on 172.16.255.250: ..... "and that MongoDB has been successfully moved to 172.16.255.251:"