envsubst.log

[vagrant@hotels_box ~]$ envsubst < docker-compose.yml 
app:
  image: python:
  command: python -v
  volumes:
    - /tmp:/workdir
[vagrant@hotels_box ~]$ python_version=alpine envsubst < docker-compose.yml 
app:
  image: python:alpine
  command: python -v
  volumes:
    - /tmp:/workdir
[vagrant@hotels_box ~]$ python_version=2.6 envsubst < docker-compose.yml 
app:
  image: python:2.6
  command: python -v
  volumes:
    - /tmp:/workdir
[vagrant@hotels_box ~]$