Specify the env file docker compose uses

Question

Is it possible to specify the env file that docker compose uses for variable substitution? Currently it defaults to ".env" but I have different flavours of compose files which should have different envs.

Answer 1

You can use inheritance for this. If you have one "base" service where you set up the environment, all of your other services can inherit from that.

Example:

version: "2"

services:
  base:
    env_file:
      - my_env.txt

  web:
    extends:
      service: base
    image: foo

  database:
    extends:
      service: base
    image: foo-db

The above example has everything in the same file, but you can also split this up into multiple files, where the base service would reside in a base.yaml file. You just need to add file: base.yaml to the extends section. Please see the documentation here.

I use this approach for setting the proxy variables for all containers. I have a proxy.yaml file that defines a proxy-app service that picks up the proxy environment variables from the shell. All of my real services extend the proxy-app service and thus inherit the environment settings from that service.

Answer 2

The --env-file command-line argument and the env_file docker-compose.yml variable specify the env file to use for the container, not for the container build. To set a different file (e.g. alt.env) for the build itself, use this:

env $(cat alt.env) docker-compose up --build

Answer 3

According to the documentation, it's now possible to load an environment file (contrary to a per-service file), docker-compose will then export the env variables defined in this env file prior to starting any service, they can then be used in the docker-compose.yml config file itself:

version: "3.7"
services:
  
  node:
    environment:
      APP_ENV: "${APP_ENV}"
      NODE_ENV: "${NODE_ENV}"
    ports:
      - "${HOST_EXPOSED_NODEJS_DEBUG_PORT}:9229"
    volumes:
      - type: bind
        source: ./project
        target: /var/www/project
        read_only: false

Since docker-compose 1.25 it's also possible to specify a custom .env file with the --env-file flag (unfortunately it's currently not possible to specify multiple .env files with the --env-file flag)