ci: add validation for generated markdown

Signed-off-by: David Karlsson <david.karlsson@docker.com>

.github/workflows/validate.yml

@@ -37,6 +37,27 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
+  # check that the generated Markdown and the checked-in files match
+  validate-md:
+    runs-on: ubuntu-20.04
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Generate
+        shell: 'script --return --quiet --command "bash {0}"'
+        run: |
+          make -f docker.Makefile mddocs
+      -
+        name: Validate
+        run: |
+          if [[ $(git diff --stat) != '' ]]; then
+            echo 'fail: generated files do not match checked-in files'
+            git --no-pager diff
+            exit 1
+          fi
+
   validate-make:
     runs-on: ubuntu-20.04
     strategy: