As of today (20211019), the latest version of Rocket.Chat is 4.0 LTS. MongoDB version 4.0.x is deprecated. So I take this chance to upgrade both to the latest version.
This is a companion discussion topic for the original entry at https://henrywithu.com/upgrade-rocket-chat-from-3-x-to-4-0-mongodb-from-4-0-to-5-0-via-docker/