docs: fix ipv6 example for Compose

[milas]

Description

Fix the example for Docker Compose to configure an IPv6 network.

This was unfortunately slightly incorrect due to a mistake in the official Docker docs being propagated, which has been fixed upstream with docker/docs#18119.

The subnet must be specified as part of ipam.config.

Refer to the official Compose Spec for more details:

• https://docs.docker.com/compose/compose-file/06-networks/#ipam

Type of change

• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[milas]

For context, with Compose v2.21.0:

services:
  whoami:
    image: traefik/whoami
    networks: [ip6net]
    ports:
      - 7654:80

networks:
  ip6net:
    enable_ipv6: true
    subnet: 2001:0DB8::/112

❯ docker compose up --wait
validating /Users/milas/.../compose.yaml: networks.ip6net Additional property subnet is not allowed

⬇️

services:
  whoami:
    image: traefik/whoami
    networks: [ip6net]
    ports:
      - 7654:80

networks:
  ip6net:
    enable_ipv6: true
    ipam:
      config:
        - subnet: 2001:0DB8::/112

❯ docker compose up --wait
[+] Running 2/2
 ✔ Network ipv6_ip6net      Created
 ✔ Container ipv6-whoami-1  Healthy

❯ curl 'http://[::1]:7654'
Hostname: 2174081f7fab
IP: 127.0.0.1
IP: ::1
IP: 172.20.0.2
IP: 2001:db8::2                      <--- ✨ 
IP: fe80::42:acff:fe14:2
RemoteAddr: 192.168.65.1:36722
GET / HTTP/1.1
Host: [::1]:7654
User-Agent: curl/8.1.2
Accept: */*

[polarathene]

I thought I had verified this in the past that as working but I must have been mistaken 😬

Thanks so much for letting us know about this! ❤️

[github-actions]

Documentation preview for this PR is ready! 🎉

Built with commit: 5762703