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

web:
  # unbracketed name
  image: $IMAGE

  # array element
  ports:
    - "${HOST_PORT}:8000"

  # dictionary item value
  labels:
    mylabel: "${LABEL_VALUE}"

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

  # escaped interpolation
  command: "$${ESCAPED}"