Fix: make Docker Hub auth conditional

Docker Hub login step is now conditional on secret existence
to avoid failures when org secrets are not yet provisioned.

Refs: CAR-77

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -69,6 +69,7 @@ jobs:
             type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
 
       - name: Log in to Docker Hub
+        if: ${{ secrets.DOCKERHUB_USERNAME != '' }}
         uses: docker/login-action@v3
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}