Create u_e-dovecot-vmail-volume.md

to mount vmail volume to another path

docs/u_e-dovecot-vmail-volume.md

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+If you want to use another folder for the vmail-volume, you can create an `docker-compose.override.yml` file and add:
+```
+volumes:
+  vmail-vol-1:
+    driver_opts:
+      type: none
+      device: /data/mailcow/vmail
+      o: bind
+```