Add the ability to run some but not all samples in docker

Also, build containers with --no-cache: distributions like ArchLinux
retire their packages very quickly, need to always use up-to-date
package databases.

Signed-off-by: Alexey Neyman <stilor@att.net>

testing/docker/common-scripts/ctng-build-sample

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+set -e
+cd
+rm -rf bld-samples
+mkdir bld-samples
+cd bld-samples
+export PATH=$HOME/inst-ctng/bin:$PATH
+ct-ng build-all ${1+CT_SAMPLES="$*"}