feat(ci): add GitHub workflow to run shellcheck

.github/workflows/ci.yml

@@ -0,0 +1,26 @@
+---
+name: CI
+
+on:
+  pull_request:
+  workflow_dispatch:
+  push:
+    branches: [main, master, prod]
+
+# The list of permissions is explained on the GitHub doc:
+# https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs
+permissions:
+  actions: read
+  contents: read
+
+jobs:
+  shellcheck:
+    name: "ShellCheck"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: Scalingo/actions/shellcheck@main
+        with:
+          shellcheck-severity: style
+          files: |
+            cmnlib.sh
+...