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

@@ -178,6 +178,13 @@ test:container_scanning:
   artifacts:
     paths: [gl-sast-container-report.json]
 
+test:shellcheck:
+  stage: test
+  extends: .rules:code-changes
+  image: koalaman/shellcheck-alpine:stable
+  script:
+    - shellcheck bin/* support/**/*
+
 .verify_job:
   stage: verify
   extends: .rules:code-changes