add crypt-vol-1 to borgmatic

Your borg'ly stored vmail is useless if you lost your encryption keys.

docs/third_party-borgmatic.md

@@ -30,6 +30,7 @@ services:
     dns: ${IPV4_NETWORK:-172.22.1}.254
     volumes:
       - vmail-vol-1:/mnt/source/vmail:ro
+      - crypt-vol-1:/mnt/source/crypt:ro
       - mysql-socket-vol-1:/var/run/mysqld/:z
       - ./data/conf/borgmatic/etc:/etc/borgmatic.d:Z
       - ./data/conf/borgmatic/state:/root/.config/borg:Z
@@ -244,4 +245,4 @@ To fetch the keyfile run:
 docker-compose exec borgmatic-mailcow borg key export --paper user@rsync.net:mailcow
 ```
 
-Where `user@rsync.net:mailcow` is the URI to your repository.
+Where `user@rsync.net:mailcow` is the URI to your repository.