Comment on syntax errors (#656)

.github/workflows/lint_c.yml

@@ -43,7 +43,19 @@ jobs:
         uses: ./.github/actions/docker
 
       - name: 'Check syntax'
+        id: syntax_check
         uses: ./.github/actions/docker
-        continue-on-error: false
         with:
-          run: /syntax_check.sh
+          run: SET_GH_OUTPUT=1 /syntax_check.sh
+
+      - name: Report syntax errors
+        if: failure() && steps.syntax_check.outputs.errors && github.event.pull_request
+        uses: peter-evans/create-or-update-comment@v1
+        with:
+          issue-number: ${{ github.event.pull_request.number }}
+          body: |
+            Please fix following syntax errors:
+            ```
+            ${{ steps.syntax_check.outputs.errors }}
+            ```
+            You might want to run `docker compose exec dev /syntax_check.sh` for an auto-fix.