Add a soundness check to lint YAML files

Author: ahoppen

.github/workflows/configs/yamllint-config.yml

@@ -0,0 +1,7 @@
+extends: default
+
+rules:
+  line-length: false
+  document-start: false
+  truthy:
+    check-keys: false  # Otherwise we get a false positive on GitHub action's `on` key

.github/workflows/soundness.yml

@@ -51,6 +51,14 @@ on:
         type: string
         description: "Container image for the shell check job. Defaults to latest Swift Ubuntu image."
         default: "swift:6.0-noble"
+      yamllint_enabled:
+        type: boolean
+        description: "Boolean to enable the YAML lint job. Defaults to true."
+        default: true
+      yamllint_container_image:
+        type: string
+        description: "Container image for the YAML job. Defaults to latest Swift Ubuntu image."
+        default: "swift:6.0-noble"
 
 ## We are cancelling previously triggered workflow runs
 concurrency:
@@ -177,3 +185,21 @@ jobs:
         run: |
           apt-get -qq update && apt-get -qq -y install shellcheck
           git ls-files -z '*.sh' | xargs -0 shellcheck
+
+  yaml-lint:
+    name: Lint YAML
+    if: ${{ inputs.yamllint_enabled }}
+    runs-on: ubuntu-latest
+    container:
+      image: ${{ inputs.yamllint_container_image }}
+    timeout-minutes: 5
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - name: Run yamllint
+        run: |
+          apt-get -qq update && apt-get -qq -y install yamllint
+          curl -s https://raw.githubusercontent.com/swiftlang/github-workflows/refs/heads/main/.github/workflows/utils/yamllint-config.yml > /tmp/yamllint-config.yml
+          yamllint --strict --config-file /tmp/yamllint-config.yml ${GITHUB_WORKSPACE}