Как сделать default переменную docker-compose?

version: '3.4'
x-variables: &config
  version: ${TAG}
services:
  postgres:
    image: postgres:${web:-$TAG}
    restart: always
    environment:
      <<: *config
    volumes:
      - "fastgate_db:/var/lib/postgresql/data"
    ports:
      - "5432:5432"

TAG=6.1.0
web=DEVELOP