webserver: Use SIGKILL instead of SIGTERM to kill the container

See https://github.com/SergioBenitez/Rocket/issues/180.

configuration/services/webserver.nix

@@ -29,6 +29,9 @@
         ];
         ExposedPorts = { "3002" = { }; };
         User = "${uid}:${gid}";
+
+        # Will become SIGTERM with the next rocket release
+        StopSignal = "SIGKILL";
       };
     };