Add Shellcheck GitHub action

Noticed as missing in https://github.com/PrivateBin/docker-nginx-fpm-alpine/pull/128 and thus added.

.github/workflows/shellcheck.yml

@@ -0,0 +1,18 @@
+on:
+  push:
+    branches:
+      - master
+
+name: "Shellcheck"
+permissions: {}
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    
+    steps:
+      - uses: actions/checkout@v3
+      
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master