Fix Docker build

Author: ekilmer

.github/workflows/vcpkg_docker.yml

@@ -66,7 +66,8 @@ jobs:
         id: build
         uses: docker/build-push-action@v5
         with:
-          context: .
+          context: docker
+          file: docker/Dockerfile.ubuntu.vcpkg
           platforms: ${{ matrix.platform }}
           labels: ${{ steps.meta.outputs.labels }}
           outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true