Add workflow for validating runner scripts with shellcheck

I am about to revisit #1517, #1454, #1561, and #1560 as a part of our on-going effort to a major enhancement to the runner entrypoints being made in #1759.

This change is a counterpart of #1852. #1852 enables you to easily run shellcheck locally. This enables you to automatically run shellcheck on every pull request.

Currently, any shellcheck error does not result in failing the workflow job. Once we addressed all the shellcheck findings, we can flip the fail_on_error option to true and let jobs start failing on pull requests that introduces invalid or suspicious bash code.

.github/workflows/validate-runner-scripts.yaml

@@ -0,0 +1,29 @@
+name: Validate Runner Scripts
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+    paths:
+      - 'runner/**.sh'
+      - 'runner/**.bash'
+jobs:
+  shellcheck:
+    name: runner / shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: shellcheck
+        uses: reviewdog/action-shellcheck@v1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          reporter: shellcheck
+          path: "./runner"
+          pattern: |
+            *.sh
+            *.bash
+          exclude: "./.git/*"
+          check_all_files_with_shebangs: "false"
+          # Set this to "true" once we addressed all the shellcheck findings
+          fail_on_error: "false"