Add custom configuration to 404 hosts

[miguelangel-nubla]

Just like every other server type, support custom configuration files in 404 hosts.

[nginxproxymanagerci]

Docker Image for build 1 is available on
DockerHub
as nginxproxymanager/nginx-proxy-manager-dev:pr-4247

Note: ensure you backup your NPM instance before testing this image! Especially if there are database changes
Note: this is a different docker image namespace than the official image