Docker-Compose django-channels multichat example which allows scalable (async) workers

docker-compose.yml

# Example for a scalable worker for Andrew Goodwins django-channels multichat example
# from https://www.excella.com/insights/scaling-django-channels-with-docker
# 
# Replace the existing docker-compose.yml at 
# https://github.com/andrewgodwin/channels-examples/tree/master/multichat
# 
# Follow the docker-compose steps for db migration, etc. and then
#
#     $ docker-compose ps 
#     $ docker-compose scale worker=5 &
#     $ docker-compose ps

version: "2"

services:
  redis:
    image: redis:latest
  web:
    build: .
    command: daphne -b 0.0.0.0 -p 8000 multichat.asgi:channel_layer
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    links:
      - redis
  worker:
    build: .
    command: python manage.py runworker
    volumes:
      - .:/code
    links:
      - redis

In the web service, the command call to Daphne defines than the asgi should call a variable channel_layer, where does that variable is declared inside the ASGI configuration file?