How to set redis DB ID in Docker container?

@sam is your warning :fire: from 2018 about the message bus exclusively linked to Redis db 0 still accurate, or can we use now a different redis instance database ID to host different discourse instances?

I noticed in the multisite-configuration-with-docker howto, there is a db_id: 2 defined, I thought that may refer to the redis database ID, thus this inquiry :face_with_monocle:

           adapter: postgresql
           database: b_discourse
           pool: 25
           timeout: 5000
           db_id: 2