I've concocted this project to alleviate the confusion that surround the use of environment variables with docker-compose files.

It's my attempt at clarifying things following the confusion in my head from those Docker issues:

• WARNING: The X variable is not set. Defaulting to a blank string. #4189
• Add support for extends feature in Compose v3 / docker stack deploy#31101

$ docker-compose up test
$ docker-compose up prod
$ docker-compose up magic
$ FROM_CLI=wow docker-compose up magic
$ docker-compose up black-magic
$ docker-compose -f docker-compose.yml -f docker-compose.staging.yml up test
$ docker-compose -f docker-compose.yml -f docker-compose.staging.yml up prod
$ docker-compose -f docker-compose.yml -f docker-compose.staging.yml up magic
$ FROM_CLI=wow docker-compose -f docker-compose.yml -f docker-compose.staging.yml up magic
$ docker-compose -f docker-compose.yml -f docker-compose.staging.yml up black-magic

you can also run docker-compose config to display the compose-file with the in-container variables (specfied with environment: and env_file:) resolved.

you can also set the COMPOSE_FILE environment variable if you don't want to type all the compose file names:

$ export COMPOSE_FILE=docker-compose.yml:docker-compose.staging.yml
$ docker-compose up black-magic

to test the environment variables "inheritance" feature (to alleviate the disappearance of extends):

$ docker-compose up queen-bee worker-bee sentinel-bee

it uses docker-compose format version 3.7 which requires Docker Engine 18.06.0

Feel free to change the "version:" property in the docker-compose.yml to support older version or to explore behaviour with previous versions.