docker compose "version 3.6" does not support platform

[jetersen]

Description of the issue

Docker compose version 3.6 does not support platform yet 2.4 does? What gives?
#5767 seems only to have added it for 2.4 😭

Context information (for bug reports)

Output of "docker-compose version"
docker-compose version 1.21.2, build a1334711
docker-py version: 3.3.0
CPython version: 3.6.4
OpenSSL version: OpenSSL 1.0.2k  26 Jan 2017

Output of "docker version"
Client:
 Version:      18.05.0-ce-rc1
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   33f00ce
 Built:        Thu Apr 26 00:57:53 2018
 OS/Arch:      windows/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.05.0-ce-rc1
  API version:  1.37 (minimum version 1.24)
  Go version:   go1.10.1
  Git commit:   33f00ce
  Built:        Thu Apr 26 01:07:01 2018
  OS/Arch:      windows/amd64
  Experimental: true

docker-compose file

version: '3.6'

services:
  jenkins:
    platform: linux
    env_file: prod.env
    container_name: jenkins
    image: jenkins-master
    build: .
    ports:
      - 8080:8080
      - 50000:50000
    volumes:
      - jenkins:/var/jenkins_home
    secrets:
      - id_rsa

volumes:
  jenkins:

secrets:
  id_rsa:
    file: .\secrets\jenkins_ssh_key

Output of "docker-compose config"
ERROR: The Compose file '.\docker-compose.yml' is invalid because:
Unsupported config option for services.jenkins: 'platform'

Steps to reproduce the issue

1. use version: '3.6' in docker-compose.yml file
2. add platform to service
3. run docker-compose config

Observed result

ERROR: The Compose file '.\docker-compose.yml' is invalid because:
Unsupported config option for services.jenkins: 'platform'

Expected result

Platform to be supported

[shin-]

• For issues with the docker stack commands and the version 3 of the Compose file, use
  https://github.com/docker/cli

[jetersen]

@shin- guess I got confused that you still carried the schema around for 3.x when I was looking at the pull request.