Base busybox container for tests

This will allow us to conduct tests with it in kuryr-libnetwork and
kuryr-kubernetes without to pull anything

Change-Id: I1c887e8d669ec131dac0a9930647b0112aae881a
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>

contrib/busybox/Dockerfile

@@ -0,0 +1,3 @@
+FROM scratch
+ADD busybox.tar.xz /
+CMD ["sh"]

contrib/busybox/build_image.sh

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+#!/bin/sh
+
+tar cv --files-from /dev/null | docker import - scratch
+
+docker build -t kuryr/busybox .