[ci] add Dockerfile linting (fixes uptake#33)

.github/workflows/linting.yml

@@ -0,0 +1,29 @@
+name: linting
+
+# only run this workflow on new commits to master
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  test:
+    name: ${{ matrix.task }}
+    runs-on: macos-latet
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - task: lint-dockerfile
+    steps:
+      - name: checkout repository
+        uses: actions/checkout@v1
+      - name: lint dockerfile
+        shell: bash
+        run: |
+          brew upgrade
+          brew install hadolint
+          hadolint Dockerfile