update docker readme

Author: benniekiss

docker/README.md

@@ -19,5 +19,7 @@ docker run -d --rm -p 80:80 -p 443:443 \
 > For SSL generation, you need to run this image in a server with proper public IP and a domain name pointing to it.
 ## Environment variables
 * ```NGINX_SSL_PORT``` Changes the port that Nginx listens to. Defaults to ```443```
+* ```NGINX_SSL_CERT``` Provide a pre-generated SSL certificate. Optional
+* ```NGINX_SSL_KEY``` Provide a pre-generated SSL certificate key. Optional
 * ```LETSENCRYPT_DOMAIN``` Enables Certbot`s client execution for the specified domain. Defaults to ```none```
 * ```LETSENCRYPT_EMAIL``` Email used in Certbot`s client execution to register the certificate request. Defaults to ```example@local```