Seconding the other user’s post, it’s just storage. You can use it temporarily for the Plex server but I highly recommend offloading that task to a real computer.
A used Optiplex is usually the go-to because it’s low power and capable of doing the job. Just make sure it has enough RAM to handle the containers you wanna put on it.
Docker is a little bit of learning but you’ll get the hang of it. I found docker-compose to be much easier to learn. Portainer or Dockge can help by giving you a GUI, if that’s your preferred style (like me).