CI/docker: specify the Dockerfile to use

Author: dannywillems

.github/workflows/docker.yaml

@@ -98,7 +98,8 @@ jobs:
         id: build
         uses: docker/build-push-action@v6
         with:
-          context: ./frontend
+          context: ./
+          file: ./frontend/Dockerfile
           platforms: ${{ matrix.arch.platform }}
           cache-from: type=gha
           cache-to: type=gha,mode=max