Update to Alpine 3.23 + PHP 8.4

[pimvanpelt]

Other small fixes:

• bump client body size in nginx.conf
• avoid chown'ing all of /var/www, only the parts we need to write to
• flattened layers in Dockerfile a bit
• added docker-compose.yml, so users can docker compose build and docker compose up their own image.

Thanks for the excellent project.