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

@@ -21,9 +21,9 @@ lint-dockerfile:
 
 lint-shell-script:
   stage: test
-  image: shellcheck:latest
+  image: koalaman/shellcheck-alpine:latest
   script:
-    - find . -name "*.sh" -execdir shellcheck {} \;
+    - find . -name "*.sh" -exec shellcheck {} \;
 
 #
 # IMAGE BUILDS/PUSHES