Using MaxMind's free GeoIP databases with the official Docker PHP image

I recently had to add support for the MaxMind free GeoIP database to a PHP container build that was based on the official Docker PHP image on Docker Hub. Unfortunately, it seems nobody else who's added this support has documented it, so I figured I'd post this so that the next poor soul who needs to implement the functionality doesn't have to spend half a day doing it!

First, you need the PHP geoip extension, which is available via PECL (note: if you can make the PHP project itself use a composer library, there are a few better/more current geoip libraries available via Packagist!). Here's how to install it in one of the php 5.6 or 7.0-apache images (note that 7.1 uses Debian Stretch instead of Jessie... but the instructions should be the same there):