note about security risk of using shared network

[0x450x6c]

Hello.

Thanks for tutorial, it was helped a lot when I'm learning the docker swarm + traefik.

I configured traefik using this tutorial + docs, and didn't pay attention to network security, and did a big mistake by using same network for all my services. (now I'm using separate network for each stack, see https://github.com/traefik/traefik/issues/8010#issuecomment-1146745456)

I think it would be helpful to note about network security in the tutorial.