How variables of config file are getting replaced by docker-compose ENV?

Question

Reading this repository I cannot understand how the variables of divolte-collector.conf inside ${?*} are getting replaced by the ENV of docker-compose. I have been reading all repository and searching in docker documentation, but whithout finding any clue.

Dockerfile

docker-compose.yml

Answer 1

Phil was kind enough to correctly answer this question. (Post an "answer" and I will happily delete this copy.)

The divolte-collector.conf file supports environment variable overrides using the ${?VAR_NAME} syntax. Compose sets the variables for the container via the environment collection in the docker-compose.yml file. See also https://docs.docker.com/compose/environment-variables