Set home to /root

Fixes https://github.com/play-with-docker/play-with-kubernetes.github.io/issues/9

dockerfiles/k8s/Dockerfile

@@ -30,5 +30,7 @@ export PS1="[\h \W]$ "' >> /root/.bash_profile
 RUN mkdir -p /root/.kube && ln -s /etc/kubernetes/admin.conf /root/.kube/config \
     && rm -f /etc/machine-id
 
+WORKDIR /root
+
 CMD systemctl start docker && systemctl start kubelet \
     && while true; do bash -l; done