Re-using environment variables in docker-compose.yml

You can reference local environment variables from within a docker-compose file. Assuming what you're wanting to do is make USER_NAME the same as DB_USER_NAME:

docker-compose.yml

version: '2'

services:
  db:
    image: example/db
    ports:
      - "8443:8443" 
    container_name: db
    hostname: db
    environment:
      - USER_NAME = ${USERNAME}
      - USER_PASSWORD = ${PASSWORD}

svc:
  image: example/svc
  depends_on:
    - db
  ports:
    - "9443:9443"
  container_name: svc
  hostname: svc
  environment:
    - DB_URL = https://db:8443
    - DB_USER_NAME = ${USERNAME}
    - DB_USER_PASSWORD = ${PASSWORD}

Then, run docker-compose like:

$ USERNAME="admin" PASSWORD="admin" docker-compose up

Alternately, for something more permanent, and easier to type on a recurring basis:

$ printf '%s\n%s\n' 'export USERNAME="admin"' 'export PASSWORD="admin"' >> ~/.bash_profile
$ source ~/.bash_profile
$ docker-compose up

The extends option can be nice but it's not supported in 3.x compose files. Other ways to go are:

1. Extension fields (compose file 3.4+)

   If you can use 3.4+ compose files, extension fields are probably the best option:

   docker-compose.yml

    version: '3.4'
   
    x-common-variables: &common-variables
      VARIABLE: some_value
      ANOTHER_VARIABLE: another_value
   
    services:
      some_service:
        image: someimage
        environment: *common-variables
   
      another_service:
        image: anotherimage
        environment:
          <<: *common-variables
          NON_COMMON_VARIABLE: 'non_common_value'
   

2. env_file directive

   docker-compose.yml

    version: '3.2'
   
    services:
      some_service:
        image: someimage
        env_file:
          - 'variables.env'
   
      another_service:
        image: anotherimage
        env_file:
          - 'variables.env'
   

   variables.env

    VARIABLE=some_value
    ANOTHER_VARIABLE=another_value
   

3. .env file in project root (or variables at actual compose environment)

   Variables from .env file can be referenced in service configuration:

   docker-compose.yml

    version: '3.2'
   
    services:
      some_service:
        image: someimage
        environment:
          - VARIABLE
   
      another_service:
        image: anotherimage
        environment:
          - VARIABLE
          - ANOTHER_VARIABLE
   

   .env

    VARIABLE=some_value
    ANOTHER_VARIABLE=another_value
   

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.