Fix slow shutdown for the Grafana container (#5746)

The root process of the container is `/bin/sh`, which ignores the
initial `SIGTERM` that Docker sends. So Docker has to wait 10 seconds
and then kill everything with `SIGKILL`.

Make Grafana itself the root process instead. Grafana handles the
`SIGTERM` and shuts down quickly.

docker-compose.yml

@@ -319,7 +319,7 @@ services:
               path:  /var/lib/grafana/dashboards
               foldersFromFilesStructure: true
         EOF
-        /run.sh
+        exec /run.sh
     networks:
       cvat:
         aliases: