host-gateway variable does not work on build time when defined in the docker-compose file

[HCl-not-HCi]

Description

I defined the host mapping "host.docker.internal:host-gateway" in the extra_hosts definition in the build definition of my docker-compose.yaml file (see below). The problem is that docker compose does not convert the host-gateway variable when starting the build. Instead, I get the error invalid host host.docker.internal:host-gateway.

The same host mapping works when using it in the service definition outside the build definition (so that it applies at runtime instead of during build time; see also in the docker-compose.yaml below).

Using the docker build command to define the host works on the other hand (see the command below).

In the following are all my files and commands to reproduce the issue:

docker-compose.yaml:

services:
  debian:
    build:
      context: .
      extra_hosts:
        - "host.docker.internal:host-gateway" # Does not work
        # - "host.docker.internal:172.17.0.1" # (host IP); works but I have to define the IP statically
    extra_hosts:
      - "host.docker.internal:host-gateway" # Works

Dockerfile:

FROM debian:bullseye

# Just to print some debug output
RUN cat /etc/hosts

CMD tail -f /dev/null

Command to build using docker compose: sudo docker compose build --no-cache --progress=plain

Command to build using docker (this one works as expected): sudo docker build . --no-cache --add-host "host.docker.internal:host-gateway"

Steps to reproduce the issue:

1. Set up a directory containing above files
2. Run the above docker compose build command and see the error
3. Use the docker build command above and see it working (the hosts file is printed during build where you can see the correct IP address)

Describe the results you received:

docker compose fails with the error invalid host host.docker.internal:host-gateway instead of replacing host-gateway with the host IP address.

Describe the results you expected:

I would expect that the image builds without errors and with host.docker.internal populated with the correct IP address

Additional information you deem important (e.g. issue happens only occasionally):

None

Output of docker compose version:

Docker Compose version v2.6.0

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Docker Buildx (Docker Inc., v0.8.2-docker)
  compose: Docker Compose (Docker Inc., v2.6.0)
  scan: Docker Scan (Docker Inc., v0.17.0)

Server:
 Containers: 4
  Running: 2
  Paused: 0
  Stopped: 2
 Images: 27
 Server Version: 20.10.17
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 0197261a30bf81f1ee8e6a4dd2dea0ef95d67ccb
 runc version: v1.1.3-0-g6724737
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.4.0-124-generic
 Operating System: Ubuntu 20.04.4 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 16
 Total Memory: 22.93GiB
 Name: <removed>
 ID: 7I44:XIU7:6NQ7:DMGQ:MRYN:FW7S:LBLX:JYV5:CQ7T:T6NU:KKLP:3JT7
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: socks5://127.0.0.1:7777
 HTTPS Proxy: socks5://127.0.0.1:7777
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No swap limit support

Additional environment details:

I have to use a SOCKS proxy (a port on the localhost forwards all requests through SSH to the proxy running on my client), that's why I need to know the host IP address during build time.