add docker install to debian

infrastructure/graduated/debian/debian.md

@@ -4,6 +4,7 @@
   - [Setup SSH](#setup-ssh)
   - [Fail2Ban](#fail2ban)
   - [Automatic Updates](#automatic-updates)
+  - [Docker](#docker)
   - [Extras](#extras)
 
 Note these instructions differentiate between an `operator` and a `server`. The operator can be
@@ -97,6 +98,10 @@ apt install -y unattended-upgrades
 systemctl enable --now unattended-upgrades.service
 ```
 
+## Docker
+
+<https://docs.docker.com/engine/install/debian/#installation-methods>
+
 ## Extras
 
 On the server: