Colin O'Dell has posted a guide that you can follow to help you test your applications with the latest version of the PHP 7.2 pre-releases using a simple Docker setup.
PHP 7.2 is slated for release in November 2017, but you don't have to wait until then to start testing your applications! We can easily use Docker to test against the latest pre-release versions of 7.2.
He's even created a Docker image that's posted on DockerHub to make the process even simpler. He shows how to reference specific types (php-fpm, apache, etc) and specific tags for the PHP pre-releases on each. He encourages you to log bugs in the PHP if found and report issues to him if there's a problem with the Docker setup. He also mentions that, since the mcrypt extension was removed by default from PHP 7.2, you'll need some special commands to get it set up and working (via a PECL install).