Add github workflow for shellcheck

.github/workflows/shellcheck.yml

@@ -0,0 +1,18 @@
+name: Run Shellcheck on push and PR
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Run ShellCheck
+      uses: ludeeus/action-shellcheck@master
+      env:
+        SHELLCHECK_OPTS: -x
+      with:
+        additional_files: bookworm bullseye switchtoTesting switchtoUnstable