[Bug]: mongo requires alternate shell in command to work as shard config server

Error Message and Logs

BadValue: Cannot start a configsvr as a standalone server. Please use the option --replSet to start the node as a replica set.

Steps to Reproduce

1. Pull a MongoDB image (version 6, 7, or 8—it doesn't matter).
2. Make the following mongod.conf:

sharding:
  clusterRole: configsvr
replication:
  replSetName: "cfg0"

3. Create a docker-compose.yml file for starting mongo similar to this (simplified for illustration):

services:
    mongocfg1:
        image: 'mongo:8'
        command: 'mongod --config /etc/mongo/mongod.conf'
        container_name: mongocfg1
        # ...
        volumes:
            - 'mongodb-configdb:/data/configdb'
            - 'mongodb-db:/data/db'
            -
                type: bind
                source: /data/mongod.conf
                target: /etc/mongo/mongod.conf
                read_only: true
volumes:
    mongodb-configdb:
        name: mongodb-configdb
        external: false
    mongodb-db:
        name: mongodb-db
        external: false

4. Start mongo with docker compose up.
5. MongoDB will not start and will complain that the replica set option is missing, even though it's clearly there in the mongod.conf.

Additional Information

After a lot of digging and experimenting, I found the strangest cause for this. It's the command line of docker-compose.yml. Whether I make it command: 'mongod --config /etc/mongo/mongod.conf' or command: --config /etc/mongo/mongod.conf, it will not read the replica set options when including the configsvr option. And it doesn't seem to matter what I change in mongod.conf.

However, if we replace the command line with:

 command: "/bin/sh -c 'exec mongod --config /etc/mongo/mongod.conf'"

Or:

entrypoint: "/bin/sh"
command: "-c 'exec mongod --config /etc/mongo/mongod.conf'"

And then run docker compose up, mongod appears to start fine.

Aside: even though the error message is complaining about missing replica set configuration, adding --replSet NAME to the command line still didn't fix it. It seems the only way to use Docker to start a shard config server is by doing the shell trick as shown above.

Although I've found this workaround, this is not the way the Docker image documentation says to do it, and it's not possible with systems that take control of docker-compose.yml (such as Coolify), forcing it back to command: 'mongod --config /etc/mongo/mongod.conf', which should work anyway!

For reference, here's the MongoDB documentation on creating a shard config server. Yes, it points out binding an IP, but I've noticed that makes no difference with this problem.

Start with the shown mongod.conf and docker-compose.yml command and compare how the Docker image starts mongod with and without the /bin/sh -c workaround. Check the image entrypoint behavior and reproduce the config-server startup failure on MongoDB 6, 7, or 8. Done means the documented command starts a config server using the replica-set settings without requiring the shell workaround.