Allow ENV variables in docker-compose.yml

[mattes]

docker-compose.yml

web:
  image: some-image
  volumes:
    - $PWD:/home/web

It would be really cool if env variables would get replaced with their actual values during docker-compose up.

[mattes]

ref: #939

[hauleth]

👍

It will allow tools like invoker to run Composer apps in background.

[pwaller]

Looks like a duplicate of #495

[aanand]

Closing in favour of #1377