Docker compose host variable in extra_hosts not working

I have a docker compose file with .env file, for example the .env file has this HOSTNAME=a1.test.com, and a docker-compose file has this:

...
extra_hosts:
  - "${HOSTNAME}:1.1.1.1"
  - "test.com:1.1.1.1"

When I run the container the /etc/hosts file content is:

1.1.1.1          
1.1.1.1    test.com

So the docker compose not putting the variable data during the run process.

I tried this hostname: "${HOSTNAME}" and it is working fine the host name of the container is correct a1.test.com.

Any idea how can achieve what I want?

I've tried your case, and it works fine on my side:

version: '2'
services:
  test:
    image: ubuntu:16.04
    command: sleep 9999
    extra_hosts:
      - "${HOSTNAME}:1.1.1.1"
      - "test.com:1.1.1.1"

and here is what I get in the container

cat /etc/hosts
...
1.1.1.1 a1.test.com
1.1.1.1 test.com

(edit: indentation)

StackExchange DevOps Q#2392, answer score: 4