GH Actions/validate: add shellcheck job

As this repo now contains bash scripts for the end-to-end tests, let's also run a minimal QA check on the code of the shell scripts.

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

Author: jrfnl

.github/workflows/validate.yml

@@ -150,3 +150,21 @@ jobs:
     if: ${{ github.event_name != 'schedule' || github.event.repository.fork == false }}
 
     uses: PHPCSStandards/.github/.github/workflows/reusable-remark.yml@main
+
+  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/[email protected]
+        with:
+          format: gcc

.shellcheckrc

@@ -0,0 +1,5 @@
+shell=bash
+color=always
+
+external-sources=false
+source-path=/tests/EndToEnd