Add new `docker-php-ext-*` scripts for magic

For example, the following makes WordPress work:

```Dockerfile
RUN apt-get update && apt-get install -y libpng12-dev && rm -rf /var/lib/apt/lists/* \
	&& docker-php-ext-install gd \
	&& apt-get purge --auto-remove -y libpng12-dev

RUN docker-php-ext-install mysqli
```