docker run -p 30001:27017 --name mongodb mongo:3.4 mongod --replSet rs 
docker exec -it mongodb mongo --eval "rs.initiate()"

2.3.3 :001 > gem 'mongo','2.4.3'; require 'mongo'; client = Mongo::Client.new(['127.0.0.1:30001'])
DEBUG -- : MONGODB | Topology type 'unknown' initializing.
DEBUG -- : MONGODB | Server 127.0.0.1:30001 initializing.
DEBUG -- : MONGODB | Server description for 127.0.0.1:30001 changed from 'unknown' to 'primary'.
DEBUG -- : MONGODB | Server xxx:27017 initializing.
DEBUG -- : MONGODB | getaddrinfo: nodename nor servname provided, or not known
DEBUG -- : MONGODB | Topology type 'unknown' changed to type 'replica set'.
DEBUG -- : MONGODB | There was a change in the members of the 'replica set' topology.
 => #<Mongo::Client:0x70171873xxxxxx cluster=127.0.0.1:30001, xxx:27017>

2.3.3 :001 > gem 'mongo','2.5.3'; require 'mongo'; client = Mongo::Client.new(['127.0.0.1:30001'])
DEBUG -- : MONGODB | Topology type 'unknown' initializing.
DEBUG -- : MONGODB | Server 127.0.0.1:30001 initializing.
DEBUG -- : MONGODB | Server description for 127.0.0.1:30001 changed from 'unknown' to 'unknown'.
DEBUG -- : MONGODB | Server xxx:27017 initializing.
DEBUG -- : MONGODB | getaddrinfo: nodename nor servname provided, or not known
DEBUG -- : MONGODB | There was a change in the members of the 'unknown' topology.
DEBUG -- : MONGODB | There was a change in the members of the 'unknown' topology.
 => #<Mongo::Client:0x70113311xxxxxx cluster=127.0.0.1:30001, xxx:27017>