I read somewhere that v9 is supposed to be compatible with v8 but not guaranteed. In you case since you want to run containers, it would only mean a binary built for v8 could possibly run in a container made for v9, but Docker would see that the architecture is different and wouldn’t run the container without emulating v9. Since Docker Desktop can emulate architectures, you could probably run containers from images built for v8, but emulation is never perfect so in some cases you could run into some issues. How frequently, I don’t know. You could open an issue in the github repo to ask for images built for ARM64v9. Although it is not an actual issue at the moment, it is not about Docker itself but official images so I would probbaly not ask for it in the roadmap. If I’m wrong, they wil redirect you to the roadmap.
But to reply directly to your original question, I have no information about whether arm64v9 is planned or not, but I’m pretty sure it will happen if new Macs will all have this CPU.