Add build comparison to GitHub Actions workflow

Include steps to compare current code with previous commit using Docker builds. Generate a diff for HTML files and upload artifacts for review.

.github/workflows/ci-build-site.yml

@@ -27,12 +27,63 @@ on:
 
 jobs:
   build:
-    name: build
+    name: build-and-compare
     runs-on: ubuntu-latest
     steps:
+      - name: check out code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0  # Needed to fetch all history for comparison
 
-    - name: check out code
-      uses: actions/checkout@v4
+      - name: Get comparison commit
+        id: get-comparison-commit
+        run: |
+          if [[ "${{ github.event_name }}" == "pull_request" ]]; then
+            echo "compare_sha=$(git merge-base ${{ github.event.pull_request.base.sha }} ${{ github.event.pull_request.head.sha }})" >> $GITHUB_OUTPUT
+          else
+            echo "compare_sha=$(git rev-parse HEAD^1)" >> $GITHUB_OUTPUT
+          fi
 
-    - name: build site image
-      run: docker build -t radio-t/site .
+      - name: Build previous version
+        run: |
+          git checkout ${{ steps.get-comparison-commit.outputs.compare_sha }}
+          docker build -t radio-t/site:previous .
+          docker run --rm -v ${{ github.workspace }}/public-prev:/srv/hugo/public radio-t/site:previous
+
+      - name: Build current version
+        run: |
+          git checkout ${{ github.event.pull_request.head.sha || github.sha }}
+          docker build -t radio-t/site:current .
+          docker run --rm -v ${{ github.workspace }}/public-current:/srv/hugo/public radio-t/site:current
+
+      - name: Generate diff
+        run: |
+          # Create a directory for the diff
+          mkdir -p site-diff
+          
+          # Generate diff for HTML files
+          diff -r -N -u public-prev public-current > site-diff/changes.diff || true
+          
+          # Generate a summary of changed files
+          echo "Changed files:" > site-diff/summary.txt
+          find public-current -type f -exec sh -c '
+            prev_file="public-prev/${1#public-current/}"
+            if [ ! -f "$prev_file" ]; then
+              echo "New: ${1#public-current/}"
+            elif ! cmp -s "$1" "$prev_file"; then
+              echo "Modified: ${1#public-current/}"
+            fi
+          ' sh {} \; >> site-diff/summary.txt
+          
+          find public-prev -type f -exec sh -c '
+            current_file="public-current/${1#public-prev/}"
+            if [ ! -f "$current_file" ]; then
+              echo "Deleted: ${1#public-prev/}"
+            fi
+          ' sh {} \; >> site-diff/summary.txt
+
+      - name: Upload diff artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: site-diff
+          path: site-diff/