Add shellcheck CI job

What does this Merge Request do?

Adds a CI job for ShellCheck, and fixes existing violations.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

.gitlab-ci.yml

@@ -183,7 +183,7 @@ test:shellcheck:
   extends: .rules:code-changes
   image: koalaman/shellcheck-alpine:stable
   script:
-    - shellcheck
+    - shellcheck bin/ support/
 
 .verify_job:
   stage: verify