src:iptables: add salsa CI integration

Add salsa CI integration. Following docs at:

https://salsa.debian.org/salsa-ci-team/pipeline/blob/master/README.md

Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>

debian/gitlab-ci.yml

+include:
+ - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/salsa-ci.yml
+ - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/pipeline-jobs.yml
+
+variables:
+ RELEASE: 'unstable'