Docker compose mongo: Overriding entrypoint

I am trying to setup a MongoDB with replicaset with Docker Compose:

services:
  mongo1:
    image: mongo:6
    container_name: mongo-rs-primary
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: password
      MONGO_INITDB_DATABASE: admin
    volumes:
      - ./_docker/volumes/mongo1:/data/db
      - ./_docker/mongo-rs-key:/data/mongo-rs-key
    ports:
      - 27017:27017
    entrypoint:
      - bash
      - -c
      - |
        chmod 400 /data/mongo-rs-key
        chown 999:999 /data/mongo-rs-key
        exec docker-entrypoint.sh $$0
    command: --replSet rs0 --keyFile /data/mongo-rs-key --port 27017

  mongo2:
    image: mongo:6
    container_name: mongo-rs-replica1
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: password
      MONGO_INITDB_DATABASE: admin
    volumes:
      - ./_docker/volumes/mongo2:/data/db
      - ./_docker/mongo-rs-key:/data/mongo-rs-key
    ports:
      - 27018:27018
    entrypoint:
      - bash
      - -c
      - |
        chmod 400 /data/mongo-rs-key
        chown 999:999 /data/mongo-rs-key
        exec docker-entrypoint.sh $$0
    command: --replSet rs0 --keyFile /data/mongo-rs-key --port 27018

  mongo3:
    image: mongo:6
    container_name: mongo-rs-replica2
    environment:
      MONGO_INITDB_ROOT_USERNAME: root
      MONGO_INITDB_ROOT_PASSWORD: password
      MONGO_INITDB_DATABASE: admin
    volumes:
      - ./_docker/volumes/mongo3:/data/db
      - ./_docker/mongo-rs-key:/data/mongo-rs-key
    ports:
      - 27019:27019
    entrypoint:
      - bash
      - -c
      - |
        chmod 400 /data/mongo-rs-key
        chown 999:999 /data/mongo-rs-key
        exec docker-entrypoint.sh $$0
    command: --replSet rs0 --keyFile /data/mongo-rs-key --port 27019

Note that I needed to override the ownership of the security key file and I found online this is one way to do it, however, the weird thing is now my replica set name is --auth and not rs0? I suspect its because of exec docker-entrypoint.sh $$0 but whats wrong? I copied that from somewhere, seems like its using --auth in --replSet instead of rs0

I suspect you're only passing the first --replSet option to the image's entrypoint script. I might change the place you pass on the option from $0 to "$@", making sure to pass all the options and to correctly quote it, and then insert an extra argument to be the (now unused) $0 value.

entrypoint:
  - /bin/sh
  - -c
  - |
    ...
    exec docker-entrypoint.sh "$$@"    
  - entrypoint-script
command: --replSet rs0 ...

The syntax of sh -c is

sh -c command_string command_name argument ...

where command_name becomes the $0 value inside command_string, and the arguments become $1, $2, and so on. So adding an explicit value for the command_name argument should help resolve the syntax.

In your original formation, this gets put together as:

1. Compose parses the YAML;
2. Compose replaces $$ with $ inside the string;
3. The string-format command: is split into words;
4. The overridden entrypoint: and command: are concatenated together.

This gives you a command like

sh -c "...\nexec docker-entrypoint.sh $0\n" --replSet rs0 ...

Notice in this command we're only inserting the single parameter $0, and the option in that position is --replSet. The remaining options just get ignored.

I suspect the original pattern you had found would work if you could force Compose to make the command: into a single string, maybe by embedding it into a YAML list command: [--replSet rs0 ...]. Then the unquoted parameter expansion $0 would do word splitting after evaluating the parameter value. It's usually not a good practice in shell scripts to rely on this, though.