`docker compose up -d` then head over to http://ip:3000

compose.yaml

volumes:
  mongodb_data: { driver: local }

services:
  rocketchat:
    image: registry.rocket.chat/rocketchat/rocket.chat:${RELEASE:-latest}
    restart: always
    environment:
      MONGO_URL: "${MONGO_URL:-\
        mongodb://${MONGODB_ADVERTISED_HOSTNAME:-mongodb}:${MONGODB_INITIAL_PRIMARY_PORT_NUMBER:-27017}/\
        ${MONGODB_DATABASE:-rocketchat}?replicaSet=${MONGODB_REPLICA_SET_NAME:-rs0}}"
      MONGO_OPLOG_URL: "${MONGO_OPLOG_URL:\
        -mongodb://${MONGODB_ADVERTISED_HOSTNAME:-mongodb}:${MONGODB_INITIAL_PRIMARY_PORT_NUMBER:-27017}/\
        local?replicaSet=${MONGODB_REPLICA_SET_NAME:-rs0}}"
      ROOT_URL: ${ROOT_URL:-http://localhost:${HOST_PORT:-3000}}
      PORT: ${PORT:-3000}
      DEPLOY_METHOD: docker
      DEPLOY_PLATFORM: ${DEPLOY_PLATFORM}
    command:
      - sh
      - -c
      - |
        export INSTANCE_IP=$$(hostname -I)
        node main.js
    depends_on:
      mongodb:
        condition: service_started

  rocketchat2:
    image: registry.rocket.chat/rocketchat/rocket.chat:${RELEASE:-latest}
    restart: always
    environment:
      MONGO_URL: "${MONGO_URL:-\
        mongodb://${MONGODB_ADVERTISED_HOSTNAME:-mongodb}:${MONGODB_INITIAL_PRIMARY_PORT_NUMBER:-27017}/\
        ${MONGODB_DATABASE:-rocketchat}?replicaSet=${MONGODB_REPLICA_SET_NAME:-rs0}}"
      MONGO_OPLOG_URL: "${MONGO_OPLOG_URL:\
        -mongodb://${MONGODB_ADVERTISED_HOSTNAME:-mongodb}:${MONGODB_INITIAL_PRIMARY_PORT_NUMBER:-27017}/\
        local?replicaSet=${MONGODB_REPLICA_SET_NAME:-rs0}}"
      ROOT_URL: ${ROOT_URL:-http://localhost:${HOST_PORT:-3000}}
      PORT: ${PORT:-3000}
      DEPLOY_METHOD: docker
      DEPLOY_PLATFORM: ${DEPLOY_PLATFORM}
    command:
      - sh
      - -c
      - |
        export INSTANCE_IP=$$(hostname -I)
        node main.js
    depends_on:
      mongodb:
        condition: service_started

  mongodb:
    image: docker.io/bitnami/mongodb:${MONGODB_VERSION:-5.0}
    restart: always
    volumes:
      - mongodb_data:/bitnami/mongodb
    environment:
      MONGODB_REPLICA_SET_MODE: primary
      MONGODB_REPLICA_SET_NAME: ${MONGODB_REPLICA_SET_NAME:-rs0}
      MONGODB_PORT_NUMBER: ${MONGODB_PORT_NUMBER:-27017}
      MONGODB_INITIAL_PRIMARY_HOST: ${MONGODB_INITIAL_PRIMARY_HOST:-mongodb}
      MONGODB_INITIAL_PRIMARY_PORT_NUMBER: ${MONGODB_INITIAL_PRIMARY_PORT_NUMBER:-27017}
      MONGODB_ADVERTISED_HOSTNAME: ${MONGODB_ADVERTISED_HOSTNAME:-mongodb}
      MONGODB_ENABLE_JOURNAL: ${MONGODB_ENABLE_JOURNAL:-true}
      ALLOW_EMPTY_PASSWORD: ${ALLOW_EMPTY_PASSWORD:-yes}

  proxy:
    image: nginx
    volumes:
      - ./nginx.conf:/etc/nginx/conf.d/nginx.conf:ro
    depends_on:
      rocketchat:
        condition: service_started
      rocketchat2:
        condition: service_started
    ports:
      - 3000:80

nginx.conf

upstream rocket_goes_boooo {
	server rocketchat:3000;
	server rocketchat2:3000;
}

server {
	listen 80 default_server;
  	server_name _;
	location / {
		proxy_set_header Upgrade $http_upgrade;
    		proxy_set_header Connection "Upgrade";
    		proxy_set_header Host $host;
		proxy_pass http://rocket_goes_boooo;
	}
}

Had to modify the nginx config for it to work. Maybe outdated compared to the version when you first wrote it? Anyway, here goes:

events {}

http {
    upstream rocketchat {
        server rocketchat:3000;
        server rocketchat2:3000;
    }

    server {
        listen 80 default_server;
        server_name _;
        location / {
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "Upgrade";
            proxy_set_header Host $host;
            proxy_pass http://rocketchat;
        }
    }
}