add v4 docker steps

[soratidus999]

update readme steps to have docker config as alternative to supervisor for aav4

Update base image to have bot_conf.py mapped
x-allianceauth-base:
  # image: ${AA_DOCKER_TAG?err}
  &allianceauth-base
  build:
    context: .
    dockerfile: custom.dockerfile
    args:
      AA_DOCKER_TAG: ${AA_DOCKER_TAG?err}
  restart: always
  env_file:
    - ./.env
  volumes:
...
    - ./conf/bot_conf.py:/home/allianceauth/myauth/bot_conf.py
...

  allianceauth_discordbot:
    container_name: allianceauth_discordbot
    <<: [ *allianceauth-base ]
    restart: on-failure:1
    entrypoint: [ "python", "/home/allianceauth/myauth/bot_conf.py" ]