There's currently race conditions in the cluster.sh script: sometimes docker containers don't get IP allocated in a timely fashion, or the SSH server takes more time than expected.
Use a retry function to wait until conditions are met

I've launched your YARN cluster successfully. I'd like to copy some files to the HDFS. But I cannot log on the machine via SSH. The server asks me about the root password. I tried no password and also tried "root", "123456", "docker". None of them works. What is the root password actually? Any hint is appreciated.