Use `php -S` instead of nginx + fpm ?

[pmartin]

Once docker is available for Linux / Windows / Mac, we won't need the VirtualBox/Vagrant VM anymore to run it: the same docker run and docker build commands should work, the same way, on all three OSes.

Dropping nginx + fpm (and using php -S instead, as it's some development project) for the web form + result page would allow for an easier setup... And this might allow us to drop the VirtualBox/Vagrant requirement, running everything locally.

We can still keep the Vagrantfile (or, at least, nginx+fpm config) as a demo, just in case?