Fix public path resolution with symlinks

[mvorisek]

see #1727 (comment) discussion and analysis there

RUN mkdir /mount && mkdir /mount/html && chmod 0777 /mount/html
RUN ln -s /mount/html /var/www/ht # /var/www/html directory cannot be removed because of Docker

RUN cd /var/www/ht/ && pwd && php -r 'var_dump(getcwd());' && php demos/test.php

and test.php:

<?php var_dump(__FILE__); var_dump(__DIR__);

demo shows calling realpath on __DIR__ is redundant, so the proposed solution should imply no performance drop, realpath is called only once per app/request for entry php script file