Show the 3-part port syntax more clearly
Show the 3-part port syntax more clearly

The 3-part (IP:HOSTPORT:CONTAINERPORT) syntax in `ports` has been there a long time, but isn't adequately described here.
docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io