Merge pull request #717 from jaschweder/master

add Docker to Infrastructure section

README.md

 * [PuPHPet](https://puphpet.com/) - A web tool for building PHP development virtual machines.
 * [Puppet](https://puppet.com/) - A server automation framework and application.
 * [Vagrant](https://www.vagrantup.com/) - A portable development environment utility.
+* [Docker](https://www.docker.com/) - A containerization platform.
 
 ## Virtual Machines
 *Alternative PHP virtual machines.*