Create and publish docker image for php-v8

[pinepain]

This should be a basic image with PHP 7.2 and libv8 (without debug, probably,) and php-v8 compiled. The purpose of this image to be able to evaluate php-v8 and what it could be good for and to demonstrate how to compile it and use in custom docker images.

[pinepain]

It probably has to have php-ref too to let https://github.com/pinepain/js-sandbox work out of the box

[pinepain]

Docker image with php, composer, php-ref and php-v8 is available under pinepain/php-v8 name - https://hub.docker.com/r/pinepain/php-v8/tags/