Suggest adding the description of domainname of compose

[无数]

File: 05-services.md

I tried to configure the domainname directly, for example

services:
  centos1:
    image: centos:7
    domainname: "a.b.com"
    command: "ping localhost" # keep container running
  centos2:
    image: centos:7
    command: "ping localhost"

but can't execute ping a.b.com successfully on centos2

I'm not alone in having this problem, is the documentation missing the necessary description?

[Robert Grant]

Is it worth trying a custom tldn? I'm wondering if centos2 is just looking in a different DNS server for .com.