Re-using environment variables in docker-compose.yml

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