resolve environment incrementaly, including values loaded from previous files in the list

[ndeloof]

closes docker/compose#9980

illustration example:

$ cat compose.yml 
services:
  test:
    image: alpine
    command: sh -c "env"
    env_file:
      - "base.env"
      - "extra.env"
$  cat base.env 
FOO="expected"
$  cat extra.env 
FOO="${FOO:-default}"
$  docker compose run test | grep FOO
FOO=expected

[laurazard]

LGTM 🥳