webserver: Use SIGKILL instead of SIGTERM

This is necessary because rocket 0.4 does not support SIGTERM.

configuration/services/webserver.nix

@@ -34,6 +34,10 @@
 
     ports = [ "3002:3002" ];
     volumes = [ "tlaternet-mail:/srv/mail" ];
-    extraOptions = [ "--hostname=tlater.net" ];
+    extraOptions = [
+      "--hostname=tlater.net"
+      # This can change with rocket 0.5.
+      "--stop-signal=SIGKILL"
+    ];
   };
 }