ci(pr-checks): use pr shas for dockerfile change detection

Author: tmeckel

.github/workflows/pr-checks.yml

@@ -32,7 +32,7 @@ jobs:
 
       - name: Build Docker image if Dockerfile changed
         run: |
-          if git diff --name-only origin/${{ github.base_ref }} | grep -q '^Dockerfile$'; then
+          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