Add Debian Jessie build image

This image is meant to be used to build packages for the Jessie release of Debian. See debian/nginx-build.

.gitlab-ci.yml

@@ -59,6 +59,14 @@ build-debian-jessie-build-dev:
   except:
     - master
 
+lint-debian-jessie-build-script:
+  stage: test
+  image: debian:latest
+  before_script:
+    - apt-get install -y shellcheck
+  script:
+    - shellcheck ./debian/jessie/setup.sh
+
 # PHP 7.3
 build-php-7.3-master:
   stage: deploy