FrankenPHP, Caddy as well the Mercure and Vulcain modules can be configured using the formats supported by Caddy.
In the Docker image, the Caddyfile is located at /etc/Caddyfile.
You can also configure PHP using php.ini as usual.
In the Docker image, the php.ini file is located at /usr/local/lib/php.ini.
When using the Docker image, set the CADDY_DEBUG environment variable to debug to enable the debug mode:
docker run -v $PWD:/app/public \
-e CADDY_DEBUG=debug \
-p 80:80 -p 443:443 \
dunglas/frankenphp