I can't go into details on our use case, but if it can work for you, I highly recommend the new --tmpfs flag. If you know exactly where your application writes data and are okay with it being in memory, you can reuse your containers with a simple stop and start rather than waiting for the full setup of a new container.

With runC you can mount whatever filesystem you want, but it is up to you to setup that filesystem. So yes, you would need some kind of shim to set up your filesystem.