Add field for FPM-Chroot Docroot

If Chroot FPM is selected, add a Field for Custom Docroot. When there is detected a Custom Docroot Input change FPM Pool config with new Docroot. Like Openbasedir Field no Input = Change nothing and / Custom Input = Change Docroot in Pool config

See this Thread (German) https://forum.howtoforge.de/threads/docroot-unter-chroot-fpm.12662/#post-62035