Can not set env variable in docker container

I have some shared env. vars for some containers.
So I want to build ME_CONFIG_MONGODB_URL using these 2 shared vars..

version: "3.7"

x-environment: &commonEnvironment
    MONGO_INITDB_ROOT_USERNAME: root
    MONGO_INITDB_ROOT_PASSWORD: root

services:
  mongo:
    image: mongo
    restart: always
    container_name: mongo
    ports:
      - 27017:27017
    environment:
      <<: *commonEnvironment

    volumes:
      - mongo-data:/data/db
    networks:
      - goboards-net

  mongo-express:
    image: mongo-express
    container_name: mongo_express
    restart: always
    ports:
      - 8081:8081
    environment:
      <<: *commonEnvironment
      ME_CONFIG_MONGODB_URL: mongodb://$MONGO_INITDB_ROOT_USERNAME:[email protected]
    networks:
      - goboards-net

But while running docker-compose -f docker-compose.prod.yml up –build -d
I am getting

WARNING: The MONGO_INITDB_ROOT_USERNAME variable is not set. Defaulting to a blank string.
WARNING: The MONGO_INITDB_ROOT_PASSWORD variable is not set. Defaulting to a blank string.
WARNING: The ME_CONFIG_MONGODB_URL variable is not set. Defaulting to a blank string.

Nevertheless, mongo have these vars, but mongo-express does not. Cant understand why.