How to debug hanging "Created API client, waiting for the control plane to become ready"

I wonder if there is a way I can debug and see where in "waiting for the control plane to become ready" Kubeadm init hangs. Any -verbose parameters for kubeadm? Some other log-files or recommended debug options?
I am running Ubuntu 16.04 - but I am trying to run kubeadm from within a docker container. On my host it is working.