# expect a build-time variable

ARG A_VARIABLE

# use the value to set the ENV var default

ENV an_env_var=$A_VARIABLE

# if not overridden, that value of an_env_var will be available to your containers!

cat ./Docker/api/api.env

NODE_ENV=test

​

$ cat docker-compose.yml

version: '3'

services:

  api:

    image: 'node:6-alpine'

    env_file:

     - ./Docker/api/api.env

    environment:

     - NODE_ENV=production

env_file:

     - ./.env

docker-compose has native support now for default environment variables in a file.

env_file:

      - ./config/logging.debug.env