Prepend docker backup command with docker exec

This is so that the back up is perform through host

doc/raketasks/backup_restore.md

@@ -100,7 +100,7 @@ the host, based on your installed version of GitLab:
 - GitLab 12.1 and earlier:
 
   ```shell
-  gitlab-rake gitlab:backup:create
+  docker exec -t <container name> gitlab-rake gitlab:backup:create
   ```
 
 If you're using the [GitLab Helm chart](https://gitlab.com/gitlab-org/charts/gitlab)