Add hadolint check

Author: AlistairB

.github/workflows/ci.yml

@@ -1,12 +1,12 @@
-name: Build and Smoke Test
+name: Validate
 
 on:
   pull_request:
     branches:
       - master
 
 jobs:
-  ci:
+  build-smoke-test:
     timeout-minutes: 30
     runs-on: ubuntu-latest
     name: ${{ matrix.ghc }}-${{ matrix.deb }}
@@ -24,8 +24,8 @@ jobs:
             ghc_minor: '9.2'
     steps:
       - uses: actions/checkout@v2
-      - name: docker build [${{ matrix.ghc }}]
-        uses: nick-invision/retry@v1
+      - name: build + smoke test [${{ matrix.ghc }}]
+        uses: nick-invision/retry@v2
         with:
           timeout_minutes: 8
           max_attempts: 3
@@ -39,3 +39,11 @@ jobs:
           path: official-images
       - name: run official-images tests
         run: ./official-images/test/run.sh haskell:${{ matrix.ghc }}-${{ matrix.deb }}
+
+  hadolint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: hadolint/hadolint-action@v1.6.0
+      with:
+        recursive: true

.hadolint.yaml

@@ -0,0 +1,10 @@
+ignored:
+  # https://github.com/hadolint/hadolint/wiki/DL3008
+  # Don't want to micro manage the dependency versions.
+  - DL3008
+  # https://github.com/hadolint/hadolint/wiki/DL3003
+  # Using cd /tmp is a very convenient way to clean up after a step.
+  - DL3003
+  # https://github.com/hadolint/hadolint/wiki/DL4006
+  # The set -eux; pattern is common in many of the official images. It likely is a better approach here.
+  - DL4006