MongoDB ruby driver cannot connect to the replica set.
The driver itself seems to want to resolve the host names of each individual node e.g. mongod-node-0 => 10.0.x.x

Logs from the Mongo adapter: note where "getaddrinfo" fails to resolve the hostname to the IP address.

016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.331490 #28 DEBUG – : MONGODB | Adding 10.65.178.135:28000 to the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.333658 #28 DEBUG – : MONGODB | Adding mongod-node-0:28000 to the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.335013 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.335204 #28 DEBUG – : MONGODB | Adding mongod-node-1:28000 to the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.335781 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.335950 #28 DEBUG – : MONGODB | Adding mongod-node-2:28000 to the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.348431 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.348763 #28 DEBUG – : MONGODB | Adding 10.65.178.133:28000 to the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.351169 #28 DEBUG – : MONGODB | 10.65.178.135:28000 being removed from the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.351480 #28 DEBUG – : MONGODB | Server 10.65.178.133:28000 elected as primary in pcf_repl.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.351706 #28 DEBUG – : MONGODB | Adding 10.65.178.134:28000 to the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.354136 #28 DEBUG – : MONGODB | 10.65.178.133:28000 being removed from the cluster.
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.834764 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.835866 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:30.836627 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:31.334806 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:31.335955 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:31.336675 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:31.834838 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known
2016-07-14T15:10:42.35-0700 [APP/0] OUT D, 2016-07-14T22:10:31.835991 #28 DEBUG – : MONGODB | getaddrinfo: Name or service not known

As soon as the driver adds a host, it tries to resolve it's name. This just isn't possible in a container, we can't add host entries.

Is it possible to instruct the driver to use IP addresses only?