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.

doc/.vale/gitlab/Substitutions.yml

@@ -12,5 +12,9 @@ swap:
   param: parameter
   params: parameters
   postgres: PostgreSQL
+  raketask: Rake task
+  raketasks: Rake tasks
+  rspec: RSpec
   self hosted: self-managed
   self-hosted: self-managed
+  yaml: YAML