Can't connect to vsftpd server with alpakka (akka-streams)

I'm trying to recreate the traversal example for the Alpakka FTP-Source connector with a vsftpd server in a Docker image, but can't seem to connect. Any pointers how to adjust the code would be very welcome:

FtpSettings ftpSettings = FtpSettings
  .create(InetAddress.getLocalhost())
  .withPort(21)
  .withCredentials(FtpCredentials.NonAnonFtpCredentials.create("news", "test"))
  .withBinary(true)
  .withPassiveMode(true)
  .withConfigureConnectionConsumer(
    (FTPClient ftpClient) -> {
      ftpClient.addProtocolCommandListener(
        new PrintCommandListener(new PrintWriter(System.out), true));
    });

Source<FtpFile, NotUsed> ftp = Ftp.ls("/", ftpSettings);
ftp.to(Sink.foreach(s -> LOGGER.info(s.name())));

FYI: The login information is working e.g. with filezilla.

Source.to returns a RunnableGraph, which is a 'blueprint' that you still have to 'run':

import akka.actor.ActorSystem;
import akka.stream.Materializer;
import akka.stream.ActorMaterializer;

// Create the blueprint:
RunnableGraph blueprint = ftp.to(Sink.foreach(s -> LOGGER.info(s.name())));

// Create the system to run the stream in:
ActorSystem system = ActorSystem.create();
Materializer materializer = ActorMaterializer.create(system);

// Run the stream:
blueprint.run(materializer);

You can also use the 'runWith' shorthand:

ftp.runWith(Sink.foreach(s -> LOGGER.info(s.name())), materializer);