Add a script to create the docker-ut image

It's a fork of the mkimage-busybox.sh script and it adds socat to the
image. (socat being needed to add udp support, see #33).

This script, like mkimage-busybox.sh, probably only works on
Debian/Ubuntu.
Merge pull request #1029 from lopter/master

* Hack: add a script to create the docker-ut image (busybox + socat)