Document how to customize PHP configuration (beyond supported environment variables)

[joshtrichards]

i.e. mounting/overriding/adding to /usr/local/etc/php/conf.d

• mention loading order / priority of files
• (Compose) the override via volumes approach
• (Dockerfile/building) - upstream: https://github.com/docker-library/docs/tree/master/php#configuration
• etc

[joshtrichards]

It seems there are more than a few image users that override /usr/local/etc/php entirely; this is a problem since image changes are also located there (i.e. see recent igbinary matter in #2509 + forums/etc).

We should probably document how to add individual files in there/etc so that people aren't overriding the image's config.