[ci] Implement automatic shellcheck test on GH Actions

.github/workflows/shellcheck.yml

@@ -0,0 +1,14 @@
+on: [push, pull_request]
+name: Shellcheck
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Get shellCheck
+      run: |
+        curl -Lf https://github.com/koalaman/shellcheck/releases/download/stable/shellcheck-stable.linux.x86_64.tar.xz | tar xJfv - --strip-components=1 shellcheck-stable/shellcheck -C /bin
+    - name: Run shellcheck
+      run: |
+        git ls-files --exclude='*.sh' --ignored | xargs shellcheck