Add support for Redis

[hopeseekr]

The entrypoint needs to check if Redis is enabled.

If it is,

1. Set Redis as the de facto PHP session handler.
2. Inject the Redis password into the php.ini.

[hopeseekr]

So support for Redis has been largely added in the stock dockerize-php package that this project now uses...

What is left is:

• .env.example (copy directly from dockerize-php)
• config/cache.php
• config/session.php