Update docker-compose.yml

obtain docker guid programmatically instead of using a hardcoded 998 value that may not be the same on every server

examples/grafana-monitoring/docker-compose.yml

@@ -49,7 +49,7 @@ services:
   telegraf:
     image: telegraf:latest
     restart: unless-stopped
-    user: telegraf:998 # see: https://www.influxdata.com/blog/docker-run-telegraf-as-non-root/
+    user: telegraf:$(stat -c '%g' /var/run/docker.sock) # see: https://www.influxdata.com/blog/docker-run-telegraf-as-non-root/
     container_name: telegraf
     hostname: telegraf
     dns: