A MongoDB service with the following docker-compose configuration in compose.yml:

mongo:
  image: mongo
  restart: always
  environment:
    MONGO_INITDB_ROOT_USERNAME: ${MONGO_USER:-app}
    MONGO_INITDB_ROOT_PASSWORD: ${MONGO_PASSWORD:-!ChangeMe!}
    MONGO_INITDB_DATABASE: ${MONGODB_DB:-symfony}

causes the variables MONGO_DATABASE, MONGO_DB , MONGO_NAME to be set to symfony, which is fine. However, it also sets the authSource param of MONGO_URL to symfony as shown below:

symfony var:export --multiline             
export MONGO_DATABASE=symfony
export MONGO_DB=symfony
export MONGO_HOST=127.0.0.1
export MONGO_NAME=symfony
export MONGO_PASSWORD=!ChangeMe!
export MONGO_PORT=32795
export MONGO_SCHEME=mongodb
export MONGO_SERVER=mongodb://127.0.0.1:32795
export MONGO_URL=mongodb://app:!ChangeMe!@127.0.0.1:32795/?authSource=symfony
export MONGO_USER=app
export MONGO_USERNAME=app

Without the MONGO_INITDB_DATABASE: ${MONGODB_DB:-symfony} in the docker compose file, all database-related environment variables as well as the authSource param become empty. It would be great if you could untangle that coupling.