Fix to load app's ini files after all other extensions are loaded

[leroy0211]

Fixes #538 by specifying a different conf.d directory (app.conf.d) for loading the ini files.

Renamed the files to numerical prefixes so they are loaded in the correct order. Default first, then environment specific.

After running a dev build, this is the output of php --ini

Configuration File (php.ini) Path: /usr/local/etc/php
Loaded Configuration File:         /usr/local/etc/php/php.ini
Scan for additional .ini files in: /usr/local/etc/php/conf.d:/usr/local/etc/php/app.conf.d
Additional .ini files parsed:      /usr/local/etc/php/conf.d/docker-php-ext-apcu.ini,
/usr/local/etc/php/conf.d/docker-php-ext-intl.ini,
/usr/local/etc/php/conf.d/docker-php-ext-opcache.ini,
/usr/local/etc/php/conf.d/docker-php-ext-sodium.ini,
/usr/local/etc/php/conf.d/docker-php-ext-xdebug.ini,
/usr/local/etc/php/conf.d/docker-php-ext-zip.ini,
/usr/local/etc/php/app.conf.d/10-app.ini,
/usr/local/etc/php/app.conf.d/20-app.dev.ini