fix example she-bang line

/bin/bash doesn't exist in the busybox docker image. Previously, we were
running '/bin/sh do_build.sh'. Now we're directly executing ./do_build.sh
so the she-bang line needs fixed.

example/do_build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 echo "Hello world from sample build script!"
 echo "This is running inside a docker container."