Docker compose config with Elixir artifacts not being shared with host

docker-compose.yml

version: "3.3"
services:
  db:
    image: postgres:11.1-alpine
    ports:
      - "5432:5432"
  app:
    env_file: .env
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "4000:4000"
    volumes:
      - .:/app
      - elixir-artifacts:/opt/elixir_artifacts
    depends_on:
      - db
    environment:
      MIX_DEPS_PATH: "/opt/elixir_artifacts/deps"
      MIX_BUILD_PATH: "/opt/elixir_artifacts/_build"
volumes:
  elixir-artifacts: {}