Docker Image for build 1 is available on
DockerHub
as nginxproxymanager/nginx-proxy-manager-dev:pr-4098

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

Actually, this does not work. No matter the statement proxy_set_header Host $http_host; appears in the beginning or the end, if proxy_set_header Host $host; exists it would not lead to the intended behaviour. Some more code will be needed to correct this behaviour