compose/tests/fixtures/environment-interpolation-with-defaults/docker-compose.yml

version: "2.1"

services:
  web:
    # set value with default, default must be ignored
    image: ${IMAGE:-alpine}

    # unset value with default value
    ports:
      - "${HOST_PORT:-80}:8000"

    # unset value with empty default
    hostname: "host-${UNSET_VALUE:-}"