Working on updated documentation for this, but splitting it in "enhancements to be backported" and "for next release"

@thaJeztah and @silvin-lubecki thanks for taking care of this !

@arkodg you're welcome! And sorry for the back-and-forth on the const. I didn't realise it was in the "daemon" package at the time.

@thaJeztah - Will support for host-gateway be added to docker/compose? While it works using the docker cli directly, it looks like compose has it's own IP address validation.

@jcoutch it's supported by the API / daemon, so would be able to be supported by compose if the validation is removed; can you open a ticket in the docker compose repository? (or perhaps even better; a pull request if you're familiar with Python

@jcoutch I just tested docker compose against a daemon that supports this feature;

docker-compose version

docker-compose version 1.26.0-rc3, build 46118bc5
docker-py version: 4.2.0
CPython version: 3.7.6
OpenSSL version: OpenSSL 1.1.1d  10 Sep 2019

Using this compose file:

version: '3.6'
services:
  nginx:
    image: nginx:alpine
    extra_hosts:
      - "foo:127.0.0.1"
      - "bar:127.0.0.2"
      - "host.docker.internal:host-gateway"

Trying to run it on a docker 19.03 daemon produces an error (as expected);

docker-compose up -d
Creating network "extra-hosts_default" with the default driver
Creating extra-hosts_nginx_1 ... error

ERROR: for extra-hosts_nginx_1  Cannot create container for service nginx: invalid IP address in add-host: "host-gateway"

ERROR: for nginx  Cannot create container for service nginx: invalid IP address in add-host: "host-gateway"
ERROR: Encountered errors while bringing up the project.

On a docker daemon built from master (so with support for the host-gateway feature);

docker-compose up -d

Creating network "extra-hosts_default" with the default driver
Creating extra-hosts_nginx_1 ... done

Verifying the container that's created:

docker inspect --format='{{json .HostConfig.ExtraHosts}}' extra-hosts_nginx_1

Which produces:

[
  "foo:127.0.0.1",
  "bar:127.0.0.2",
  "host.docker.internal:host-gateway"
]

And verifying that the magic host-gateway value is replaced correctly inside the container;

docker exec extra-hosts_nginx_1 cat /etc/hosts

127.0.0.1	localhost
::1	localhost ip6-localhost ip6-loopback
fe00::0	ip6-localnet
ff00::0	ip6-mcastprefix
ff02::1	ip6-allnodes
ff02::2	ip6-allrouters
127.0.0.1	foo
127.0.0.2	bar
172.18.0.1	host.docker.internal
172.20.0.2	2eb3a4602c9a