Remove Docker and install PHP on host

Docker is increasingly causing issues, particularly around networking and maintenance. With only a couple of sites now hosted on this machine, it will be much easier to simply install PHP and Composer, and manually setup and maintain these sites via the command-line.

Install PHP 8.0 with the required extensions and Composer. This should be all that is needed, with Nginx already being installed and configured and MariaDB being hosted on another machine.