borgmatic config.yaml - set the archive label meaningfully

Currently {hostname} expands to an anonymous number. Set this part of
the archive name to something we can recognise.

templates/docker-compose/volumes/borgmatic-config/config.yaml

@@ -231,7 +231,7 @@ storage:
   # pruning/checking of archives with different archive name
   # formats.
   # archive_name_format: '{hostname}-documents-{now}'
-  archive_name_format: '{hostname}-{now:%Y-%m-%dT%H:%M:%S.%f}'
+  archive_name_format: "${BORG_ARCHIVE_LABEL}-{now:%Y-%m-%dT%H:%M:%S.%f}"
   
   # Bypass Borg error about a repository that has been moved.
   # Defaults to false.