[DOCUMENTATION] Add info about installing customize PHP extensions
[DOCUMENTATION] Add info about installing customize PHP extensions

As many uses Docker just for local development and the prebuilt
Docker images as source, some infos on how to adopt the app
container can't hurt.

* inform about the official PHP docker images functionalities
* add example for two PHP extensions

mblaschke/TYPO3-docker-boilerplate,mblaschke/TYPO3-docker-boilerplate,mblaschke/TYPO3-docker-boilerplate