Re-using environment variables in docker-compose.yml

Question

Is it possible to re-use environment variables that are shared among multiple containers?

The idea is to avoid duplication, as illustrated in this example:

Answer 1

You can use the extends directive (available in compose 1.x and 2.x) to have multiple containers inherit the environment configuration from an underlying service description. For example, put the following in a file named base.yml:

version: '2'

services:
  base:
    environment:
      DB_URL: https://db:8443
      DB_USER_NAME: admin
      DB_USER_PASSWORD: admin 

Then in your docker-compose.yml:

version: '2'

services:
  container1:
    image: alpine
    command: sh -c "env; sleep 900"
    extends:
      file: base.yml
      service: base

  container2:
    image: alpine
    command: sh -c "env; sleep 900"
    extends:
      file: base.yml
      service: base
    environment:
      ANOTHERVAR: this is a test

Then inside of container1, you will see:

DB_URL=https://db:8443
DB_USER_NAME=admin
DB_USER_PASSWORD=admin

And inside of container2 you will see:

DB_URL=https://db:8443
DB_USER_NAME=admin
DB_USER_PASSWORD=admin
ANOTHERVAR=this is a test

You can obviously use extends for things other than the environment directive; it's a great way to avoid duplication when using docker-compose.