Installing a mongo replica-set in four steps:

1. docker container definition

Mongo instance which will part of the replica-set, Dockerfile:

FROM mongo
EXPOSE 27017
CMD ["mongod", "--replSet", "my-mongo-set"]

2. stack setup

Stack via docker-compose.yml:

version: "3"
services:
  mongo1:
    build: .
    ports:
      - "0.0.0.0:30001:27017"
    networks:
      - my_mongo_cluster
  mongo2:
    build: .
    ports:
      - "0.0.0.0:30002:27017"
    networks:
      - my_mongo_cluster
  mongo3:
    build: .
    ports:
      - "0.0.0.0:30003:27017"
    networks:
      - my_mongo_cluster

networks:
  my_mongo_cluster:
    driver: bridge

3. startup

Run: docker-compose build

4. setup replica set

Connect with python and initialize replica set:

>>> from pymongo import MongoClient
>>> c1 = MongoClient('mongodb://192.168.1.63:30001')
>>> c1.admin.command('replSetInitiate', {"_id" : "my-mongo-set","members" : [
{"_id" : 0,"host" : "192.168.1.63:30001"}, 
{"_id" : 1,"host" : "192.168.1.63:30002"},
{"_id" : 2,"host" : "192.168.1.63:30003"}]})

< {u'$clusterTime': {u'clusterTime': Timestamp(1604424245, 1), 
   u'signature': {u'keyId': 0L, 
                  u'hash': Binary('\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                           , 0)}}, 
   u'ok': 1.0, u'operationTime': Timestamp(1604424245, 1)}
>>>

You need to have installed pymongo(*) and connect in this case to one of the instances running. In my case, 192.168.1.63 is the local docker host network address.

(*) pip install --user pymongo

5. connecting to replica-set

To connect to replica-set and use its features you must let it know the config, like using this db-url:

mongodb://192.168.1.63:30001,192.168.1.63:30002,192.168.1.63:30003/?replicaSet=my-mongo-set

See ya!