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.

contrib/mkimage-unittest.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+# Generate a very minimal filesystem based on busybox-static,
+# and load it into the local docker under the name "docker-ut".
+
+missing_pkg() {
+    echo "Sorry, I could not locate $1"
+    echo "Try 'apt-get install ${2:-$1}'?"
+    exit 1
+}
+
+BUSYBOX=$(which busybox)
+[ "$BUSYBOX" ] || missing_pkg busybox busybox-static
+SOCAT=$(which socat)
+[ "$SOCAT" ] || missing_pkg socat
+
+shopt -s extglob
+set -ex
+ROOTFS=`mktemp -d /tmp/rootfs-busybox.XXXXXXXXXX`
+trap "rm -rf $ROOTFS" INT QUIT TERM
+cd $ROOTFS
+
+mkdir bin etc dev dev/pts lib proc sys tmp
+touch etc/resolv.conf
+cp /etc/nsswitch.conf etc/nsswitch.conf
+echo root:x:0:0:root:/:/bin/sh > etc/passwd
+echo root:x:0: > etc/group
+ln -s lib lib64
+ln -s bin sbin
+cp $BUSYBOX $SOCAT bin
+for X in $(busybox --list)
+do
+    ln -s busybox bin/$X
+done
+rm bin/init
+ln bin/busybox bin/init
+cp -P /lib/x86_64-linux-gnu/lib{pthread*,c*(-*),dl*(-*),nsl*(-*),nss_*,util*(-*),wrap,z}.so* lib
+cp /lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 lib
+cp -P /usr/lib/x86_64-linux-gnu/lib{crypto,ssl}.so* lib
+for X in console null ptmx random stdin stdout stderr tty urandom zero
+do
+    cp -a /dev/$X dev
+done
+
+tar -cf- . | docker import - docker-ut
+docker run -i -u root docker-ut /bin/echo Success.
+rm -rf $ROOTFS