docker run mongo image on a different port

You can run processes inside a container and outside on the same port. You can even run multiple containers using the same port internally. What you can't do is map the one port from the host to a container. Or in your case, map a port that is already in use to a container.

For example, this would work on your host:

services:
    webapp:
        image: myimage
        ports:
            - '3000:3000'

    mongo:
        image: 'mongo:latest'
        ports:
            - '27018:27017'

    mongo2:
        image: mongo:latest
        ports:
            - '27019:27017'

The host mongo listens on 27017. The host also maps ports 27018 and 27019 to the container mongo instances, both listening on 27017 inside the container.

Each containers has its own network namespace and has no concept of what is running in another container or on the host.

Networks

The webapp needs to be able to connect to the mongo containers internal port. You can do this over a container network which allows connections between the container and also name resolution for each service

services:
    webapp:
        image: myimage
        ports:
          - '3000:3000'
        networks:
          - myapp
        depends_on:
          - mongo

    mongo:
        image: 'mongo:latest'
        ports:
          - '27018:27017'
        networks:
          - myapp

networks:
    myapp:
        driver: bridge

From your app the url mongo://mongo:27017 will then work.

From your host need to use the mapped port and an address on the host, which is normally localhost: mongo://localhost:27018

You can tell MongoDB to listen on a different port in the configuration file, or by using a command-line parameter:

services:
  mongo:
    image: 'mongo:latest'
    command: mongod --port 27018
    ports:
        - '27018:27018'