docker-builder: switch to Ubuntu 18.04 base image (same as host OS)

docker-builder/Dockerfile

-FROM ubuntu:16.04
+FROM ubuntu:18.04
 MAINTAINER Ansgar.Burchardt@tu-dresden.de
 RUN rm -f /etc/apt/apt.conf.d/docker-gzip-indexes \
   && rm -rf /var/lib/apt/lists/*