Add Dockerfile hadolint linting via reviewdog (#3)

Author: johnboyes

.github/workflows/reviewdog.yml

@@ -0,0 +1,18 @@
+---
+name: reviewdog
+on:
+  push:
+
+jobs:
+
+  hadolint:
+    name: runner / hadolint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v2
+      - name: hadolint
+        uses: reviewdog/action-hadolint@v1
+        with:
+          github_token: ${{ secrets.github_token }}
+          reporter: github-check