[BUG] use of DOCKER_DEFAULT_PLATFORM in addition to explicit service "platform:" results in error

[tboddyspargo]

Description

Using a compose file that has a service which specifies platform: and also supplying DOCKER_DEFAULT_PLATFORM
will result in an error. The application appears to be requesting two platforms to be targeted despite the fact that an explicit platform should override (not be combine with) a default one.

I believe this is a BUG introduce by #8848, which provided support for DOCKER_DEFAULT_PLATFORM, but used append to add it to a list of platforms, instead of letting explicit platforms override the default platform.

NOTE: I do not believe this is an issue with buildkit because I'm unable to reproduce the issue with a direct docker build/docker buildx build command. It's only reproducible when using docker compose build. Hopefully I've created this issue in the right place.

Steps to reproduce the issue:

1. Create these required files in a temporary testing directory:

# docker-compose.yml
version: '3.7'
services:
  test:
    image: test
    build:
      context: ./
    platform: linux/amd64

# Dockerfile
FROM busybox:stable

2. Run the following command:

   DOCKER_DEFAULT_PLATFORM=linux/amd64 docker compose build test

3. You will see the error below.

Describe the results you received:

multiple platforms feature is currently not supported for docker driver. Please switch to a different driver (eg. "docker buildx create --use")

Describe the results you expected:
compose should invoke a docker build command that ignores DOCKER_DEFAULT_PLATFORM because, as a "default" value, it is expected to be overridden by an explicitly declared platform.

Output of docker compose version:

Docker Compose version v2.10.2

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc., v0.9.1)
  compose: Docker Compose (Docker Inc., v2.10.2)
  extension: Manages Docker extensions (Docker Inc., v0.2.9)
  sbom: View the packaged-based Software Bill Of Materials (SBOM) for an image (Anchore Inc., 0.6.0)
  scan: Docker Scan (Docker Inc., v0.19.0)

Server:
 Containers: 68
  Running: 47
  Paused: 0
  Stopped: 21
 Images: 99
 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: 2
 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: 9cd3357b7fd7218e4aec3eae239db1f68a5a6ec6
 runc version: v1.1.4-0-g5fd4c4d
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
  cgroupns
 Kernel Version: 5.10.124-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: aarch64
 CPUs: 10
 Total Memory: 19.5GiB
 Name: docker-desktop
 ID: VIB7:SF2J:RBST:3P2A:JBNY:GFYK:VW6B:MFGG:TPCZ:3UON:U4FW:7THL
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 No Proxy: hubproxy.docker.internal
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  hubproxy.docker.internal:5000
  127.0.0.0/8
 Live Restore Enabled: false

Additional Details:
My use case is that I have multiple services in my compose config file, one of which requires an explicit platform to be used. The others can be built for any target platform. We use, e.g., DOCKER_DEFAULT_PLATFORM=linux/amd64 to specify the desired target platform for services that support it, while the services with explicit platform: keys are expected to ignore DOCKER_DEFAULT_PLATFORM.