Frank de Jonge, creator and maintainer of Flysystem, recently started using Docker to test the FTP driver. On his blog he explains the why and how.

For Flysystem, an open source PHP package to deal with filesystems, I needed a way to test FTP (barf) interactions. FTP servers are notoriously bad at abiding by the spec. ... For Flysystem's FTP(d) adapter an integration test, using an actual FTP server, brought back the level of confidence it needed.