selenium v4 docker run mulitple chrome nodes - how to set ports

I am running selenium v4 beta in docker, and trying to get multiple chrome instances in one container. On the command line (Mac), I am running

docker-compose -f docker-compose-v3.yml up --scale chrome=3

using a yml file from the git selenium docker site

version: "3"
services:
  chrome:
    image: selenium/node-chrome:4.0.0-rc-1-prerelease-20210804
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
    ports:
      - "6900:5900"
  

  edge:
    image: selenium/node-edge:4.0.0-rc-1-prerelease-20210804
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
    ports:
      - "6950:5900"

  firefox:
    image: selenium/node-firefox:4.0.0-rc-1-prerelease-20210804
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
    ports:
      - "6951:5900"

  selenium-hub:
    image: selenium/hub:4.0.0-rc-1-prerelease-20210804
    container_name: selenium-hub
    ports:
      - "4442:4442"
      - "4443:4443"
      - "4444:4444"

available at https://github.com/SeleniumHQ/docker-selenium/blob/trunk/docker-compose-v3.yml.

I get an error of the sort

Bind for 0.0.0.0:6900 failed: port is already allocated

Looking for suggestions on how to get mulitple chrome nodes available within the container so I can run tests in parallel.