docker: Add Run with volume example

Supersedes https://github.com/ether/etherpad-lite/pull/3631

Co-authored-by: RaymondCavallaro <RaymondCavallaro@users.noreply.github.com>

doc/docker.md

@@ -100,3 +100,12 @@ docker run -d \
 	-e 'ADMIN_PASSWORD=supersecret' \
 	etherpad/etherpad
 ```
+
+Run a test instance running DirtyDB on a persistent volume:
+
+```
+docker run -d \
+	-v etherpad_data:/opt/etherpad-lite/var \
+	-p 9001:9001 \
+	etherpad/etherpad
+```