Update doc: specify a dummy command for "docker create"

"docker create" for data containers should specify a dummy command to
avoid accidential "docker start" to cause unexpected data loss.

Signed-off-by: Hong Xu <hong@topbug.net>

docs/sources/userguide/dockervolumes.md

@@ -124,7 +124,7 @@ Let's create a new named container with a volume to share.
 While this container doesn't run an application, it reuses the `training/postgres`
 image so that all containers are using layers in common, saving disk space.
 
-    $ sudo docker create -v /dbdata --name dbdata training/postgres
+    $ sudo docker create -v /dbdata --name dbdata training/postgres /bin/true
 
 You can then use the `--volumes-from` flag to mount the `/dbdata` volume in another container.