Add shellcheck GitHub action

.github/workflows/shellcheck.yml

@@ -0,0 +1,16 @@
+name: portzap
+permissions: {}
+
+jobs:
+  shellcheck:
+    name: shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ludeeus/action-shellcheck@master
+      - run: ./bin/run-shellcheck
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]