Docker-compose using host environment variable

Question

I'm trying to do the following in my docker-compose.yml

But I hit this warning? WARNING: The HOSTNAME variable is not set. Defaulting to a blank string

environment: KAFKA_ADVERTISED_HOST_NAME: ${HOSTNAME}

The HOSTNAME environment variable is obviously set on the host.

Answer 1

An error saying the variable is not set looks strange indeed.

I use host variables like this:

mycontainer:
  image: <repo>/<image>:${SERVICE_VERSION}
  environment:
    - DB_USER=${DB_USER}
    - DB_PASS=${DB_PASS}