Pin base image to 16.04

cmd/kured/Dockerfile

-FROM ubuntu
+FROM ubuntu:16.04
 RUN apt-get update && apt-get install -y ca-certificates && rm -rf /var/cache/apt
 ADD https://storage.googleapis.com/kubernetes-release/release/v1.7.6/bin/linux/amd64/kubectl /usr/bin/kubectl
 RUN chmod 0755 /usr/bin/kubectl