cat file into rkt run --interactive doesn't work

I'd expect

cat file | rkt run --interactive --exec=cat to cat the file.

Instead it just hangs, waiting for stdin input. i.e. stdin isn't chained from the user who runs it to the container.

So for instance, in k8s we provide a python container to run python scripts that parse yaml files (and yaml file is cat into the running container). This works fine when the python container runtime is docker, but not when the same container image is used with rkt.