Add Docker library.

README.md

@@ -838,6 +838,7 @@ Libraries to help manage database schemas and migrations.
 
 * [Ansible](https://www.ansible.com/) - A radically simple orchestration framework.
 * [Docker](https://www.docker.com/) - A containerization platform.
+* [Docker PHP Extension Installer](https://github.com/mlocati/docker-php-extension-installer) - Easily install PHP extensions in Docker containers.
 * [Laravel Homestead](https://laravel.com/docs/master/homestead) - A local development environment for Laravel. 
 * [Laradock](http://laradock.io/) - A full PHP development environment based on Docker.
 * [Puppet](https://puppet.com/) - A server automation framework and application.