I'm running a Mongodb cluster with two routers, 3 config servers as a replica set and 4 single node shard servers. The routers and config servers are running in a Docker swarm but the shards run on bare metal. My objective is to upgrade from version 3.4 to 4.2. I understand the upgrade path involves first going to version 3.6 before going to 4.2. I also understand I first need to modify the shards to run as single node replica sets.
However I'm unsure how to upgrade the config server replica set. Mongo's upgrade instructions describe upgrading the two secondary replicas and then stepping down the primary replica before upgrading it. This makes sense but I don't know how to apply these steps to a Docker swarm implementation.
Before reading Mongo's instructions I assumed I would only need upgrade to Docker image inside the yml file. Would it be as simple as that?