Docker Uses Environment Variables to Set Up Extra Hosts

I am using docker compose to run my application. For this, I need to set up the host inside the container (it depends on the environment I am running in).

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My method is:

Create an environment file and set variables:

#application.env
SERVER_IP=10.10.9.134

My docker compose file looks like:

version: '2'
services:

  api:
    container_name: myApplication
    env_file:
      -application.env
    build: ./myApplication/
    entrypoint: ./docker/api-startup.sh
    ports:
      -"8080:8080"
    depends_on:
      -redis
    extra_hosts: &extra_hosts
      myip: $SERVER_IP

But my problem is that the variable SERVER_IP is never replaced.

When I run the docker-compose configuration, I see:

services:
  api:
    build:
      context: /...../myApplication
    container_name: myApplication
    depends_on:
    -redis
    entrypoint: ./docker/api-startup.sh
    environment:
      SERVER_IP: 10.10.9.134
    extra_hosts:
      myip:''
    ports:
    -8080:8080

I tried to use $SERVER_IP or $(SERVER_IP) to replace variable references, but it didn't work.

I created a file .env, added a single line HOST = test.example.com, and then did this in docker-compose:

extra_hosts:
-myip:${HOST}

docker-compose config then shows

extra_hosts:
      myip: test.example.com

For this, I followed the documentation on .env files in Docker-compose environment variables

UPDATE

According to the Docker documentation,

Note: If your service specifies a build option, variables defined in environment files will not be automatically visible during the build.
Use the args sub-option of build to define build-time environment variables.

It basically means that if you put variables in the .env file, you can use them in docker-compose.yml to replace them, but if you use the env_file option for a specific container, you can only see the variables inside the Docker container, not In the build. It is also logical, env_file replaces docker run --env-file = FILE...and nothing else.

Therefore, you can only put your values into .env. Or, as William described, you can use the host's environment variables.