Fix: Sign artifacts on CentOS Stream 8/9

Fix: Sign artifacts on CentOS Stream 8/9

Update the sigul-sign-dir.sh to sign artifacts using docker.
The docker image is built on CentOS Streams 8/9. The newer version
of sigul 1.1.1 available for CentOS 8 is not backwords compatible
with the version of sigul on CentOS 7.

As a temporary workaround build a CentOS7 docker image with
sigul installed and use it for signing artificats on platforms
where sigul is not readly available.

Issue-ID: IT-23826
Change-Id: Ie22e23240f7fe388219c0afc4d4c229f390efa9c
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>