Add sample using docker image with a mounted volume

[alecgrieser]

We have some examples on how to use our server docker image in a few different ways, but it should also be possible to use a mounted volume to persist a local cluster between runs. (The current example loses all data after the cluster shuts down (the intended use case being for developers to use it for local testing).) If the docker compose network is set up with a mounted volume on the host, then it should work except that docker's mounted file system doesn't support direct I/O (alas), so the start up script would need to set the proper knob (which maybe is worth showing in and of itself).