Add wordpress image

[mzanibelli]

This is a Wordpress image based on PHP 7.2.5 + Apache.

We need it to be built by a compatible kernel since the latest doesn't allow the use of dpkg(1) (/!\). This is the first one of quite a lot of pull requests coming I guess.

• https://github.com/docker/for-linux/issues/480
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=836211
• https://bbs.archlinux.org/viewtopic.php?pid=1816302