docker-compose/tests/fixtures/extends/docker-compose.yml

myweb:
  extends:
    file: common.yml
    service: web
  command: sleep 300
  links:
    - "mydb:db"
  environment:
    # leave FOO alone
    # override BAR
    BAR: "2"
    # add BAZ
    BAZ: "2"
mydb:
  image: busybox
  command: sleep 300