Update docker install command (#312)

Update example/z-docker-esy Docker install command

example/z-docker-esy/README.md

@@ -78,7 +78,7 @@ $ ssh root@my-droplet
 [Install Docker](https://www.digitalocean.com/community/tutorials/how-to-install-and-use-docker-on-ubuntu-20-04):
 
 ```
-$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add -
+$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo gpg --dearmor -o /etc/apt/trusted.gpg.d/docker.gpg
 $ add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu focal stable"
 $ apt update
 $ apt install docker-ce -y