ci: Specify the shebang sequence for build.sh

This is necessary for the follow up patch, because the default
entrypoint for a Dockerfile is exec.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>

ci/build.sh

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 # This script is used to build libvirt inside the container.
 #
 # You can customize it to your liking, or alternatively use a