The MongoDB images have changed behavior, and during container startup the MongoDB server is started once, then stopped, and then restarted. Each time the server starts it outputs the following line:

2017-04-04T15:45:16.901+0000 I NETWORK  [initandlisten] waiting for connections on port 27017

Our build starts the mongo-init replica set initiator container as soon as the mongo container outputs a line containing "waiting for connections on port 27017". Since that appears once before the server is stopped, the mongo-init container gets an error upon connection and terminates immediately. As a result, the build times out waiting for the completion of the replica set initiation.