ci(pr-checks): integrate changed-files action for selective Docker builds

Leverage tj-actions/changed-files@v46 to check Dockerfile modifications,
eliminating custom git diff and ensuring image builds only for relevant PR changes

Author: tmeckel

.github/workflows/pr-checks.yml

@@ -25,17 +25,21 @@ jobs:
         uses: actions/checkout@v5
         with:
           fetch-depth: 0
+
+      - name: Detect changed files
+        id: changed-files
+        uses: tj-actions/changed-files@v46
+        with:
+          files: |
+            Dockerfile
+
       - name: Setup toolchain with mise
         uses: jdx/mise-action@v2
 
       - name: Run prek hooks
         run: prek run --all-files --show-diff-on-failure --color=always
 
       - name: Build Docker image if Dockerfile changed
+        if: steps.changed-files.outputs.any_changed == 'true'
         run: |
-          if git diff --name-only "${{ github.event.pull_request.base.sha }}" "${{ github.sha }}" | grep -q '^Dockerfile$'; then
-            echo "Dockerfile changed — building image..."
-            docker build -t opencode-cli-pr:latest .
-          else
-            echo "Dockerfile not changed — skipping build."
-          fi
+          docker build -t opencode-cli-pr:latest .