Change the process receiving stdin in docker container

I have an image whose ENTRYPOINT must be a definite process (i.e. systemd), but I also need to be able to run a bash shell in order to send commands to the container. This has to be done through the container stdin, but as far as I understand the container stdin is routed to the ENTRYPOINT process. Is there a way to re-route the stdin to another process like a bash shell spawn by a systemd unit?