Fix link to Docker's user guide (#3651)
Fix link to Docker's user guide (#3651)

* Fix link to Docker's user guide

* Update link to the Docker installation guide

stweil/letsencrypt,lmcro/letsencrypt,jsha/letsencrypt,letsencrypt/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,jsha/letsencrypt,lmcro/letsencrypt