I don't have much experience with Docker, but shouldn't /var/www/certbot/ also be mapped as a volume in the nginx container/image/whatever to work?
Also, in the nginx container/image/whatever you're mapping /etc/letsencrypt/ to /etc/letsencrypt/, but in the certbot container/image/whatever you're mapping ./certbot/conf/ to /etc/letsencrypt/? That would mean nginx and Certbot wouldn't use the same directory as /etc/letsencrypt/, right?