[#1] Remove Docker and install PHP on host

Associated issue: #1 (closed).

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.

Closes #1 (closed).