Limit journald disk usage on machines that ship logs away

nixos/modules/monitoring/exporters/promtail.nix

@@ -47,6 +47,12 @@ in {
       # for further details about these options.
       #
       MaxFileSec=1day
+
+      # This asks journald to not use more than 500M of disk space.  Due to
+      # journald's characteristics this might only be a week of logs, but that
+      # should be okay since we ship all logs to a central server that keeps
+      # them for a while longer.
+      SystemMaxUse=500M
     '';
 
     services.promtail.configuration = {