$ docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7 -t username/demo:latest --push .
error: multiple platforms feature is currently not supported for docker driver. Please switch to a different driver (eg. "docker buildx create --use")
make: *** [docker-build-tag] Error 1
The solution is simple. Create a new builder:
$ docker buildx create --name multiarch --use