(CAT-2013) Implement shellcheck in ci workflow

Following the implementation of a shellcheck action in a provision
module workflow, we have decided it is useful enough to be implemented
in our reusable gem ci workflow.

This commit adds the shellcheck action as a new job in the gem ci
workflow.

Author: LukasAud

.github/workflows/gem_ci.yml

@@ -26,6 +26,16 @@ on:
         type: "string"
 
 jobs:
+  shellcheck:
+    name: "shellcheck"
+    runs-on: ${{ inputs.runs_on }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: shellcheck
+        uses: reviewdog/action-shellcheck@v1
+        with:
+          check_all_files_with_shebangs: "true"
+
   spec:
     name: "spec"
     runs-on: ${{ inputs.runs_on }}