GH Actions/basic QA: add shellcheck job

As this repo now contains a shell script to find & replace placeholders for output snippets in the wiki files with real output, let's also run a minimal QA check on the code of the shell script.

Refs:
* https://github.com/koalaman/shellcheck
* https://github.com/koalaman/shellcheck/wiki

Author: jrfnl

.github/workflows/basic-qa.yml

@@ -78,3 +78,21 @@ jobs:
       - name: Fail the build when more spelling issues were found than expected
         if: ${{ always() && ( steps.spellcheck.outputs.number_of_issues != 5 || steps.spellcheck.outputs.number_of_files_with_issues != 3 ) }}
         run: exit 1
+
+  shellcheck:
+    name: 'ShellCheck'
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up problem matcher
+        uses: lumaxis/shellcheck-problem-matchers@v2
+        with:
+          format: gcc
+
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@2.0.0
+        with:
+          format: gcc

.shellcheckrc

@@ -0,0 +1,8 @@
+shell=bash
+color=always
+
+external-sources=false
+source-path=/build
+
+# Turn on all checks.
+enable=all