ci: disable registry cache for backend build to fix poisoned buildcache

The backend buildcache was contaminated with frontend image layers, causing
the backend image to be built with the wrong content. Using no-cache forces
a fresh build until the cache can be reliably separated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker-build-push.yml

@@ -135,8 +135,7 @@ jobs:
           push: ${{ gitea.event_name != 'pull_request' }}
           tags: ${{ steps.meta-backend.outputs.tags }}
           labels: ${{ steps.meta-backend.outputs.labels }}
-          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}:buildcache
-          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.BACKEND_IMAGE_NAME }}:buildcache,mode=max
+          no-cache: true
           build-args: |
             NODE_ENV=production
             CI=true