edX forum mongodb authentication

Interesting. I suggest you pass the MONGOID_AUTH_MECH environment variable to the forum container. You can do this by defining a custom docker-compose.override.yml file:

$ cat "$(tutor config printroot)/env/local/docker-compose.override.yml"
version: "3.7"
services:
  forum:
    environment:
      - "MONGOID_AUTH_MECH=:scram"