Set extra host in environment variables - docker

Question

I'm using docker compose to run my application. And for do that I need to set the hosts inside container (it's depends on the environment i'm running).
My approach was:
Create an environment file and set the variable:
#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 docker-compose config 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've tried to replace the variable reference using $SERVER_IP or ${SERVER_IP} but it didn't work.

Answer 1

I created a file .env, added single line HOST=test.example.com, then did this in docker-compose:
extra_hosts:
- myip:${HOST}
docker-compose config then shows
extra_hosts:
myip: test.example.com
To do this I followed the documentation from Docker-compose environment variables the section about .env file
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 if you place your variables in .env file, you can use them for substitution in docker-compose.yml, but if you use env_file option for the particular container, you can only see the variables inside the Docker container, not during the build. It is also logical, env_file replaces docker run --env-file=FILE ... and nothing else.
So, you can only place your values into .env. Alternatively, as William described, you can use host's environment variables.

Answer 2

EDIT
Try the following:
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:
- "myip:${SERVER_IP}"
Ensure curly bracers and that the environment variable exists on the host os.