OBSOLETE: Using Traefik is a much better option
Simple certbot modified image to renew domains with letsencrypt every month
After you enabled the ACME challenge support for your server (e.g. for nginx and your letsencron image is running under the service name letsencryt simply run (if using docker-compose)
docker-compose exec letsencrypt add_domain www.example.com
The renewal command is called hourly and randomly as suggested on the official documentation