Change default mountpoints to play nice with Container-Optimized OS

[endertekin]

Hi - I was able to install and run the service, and create a persistent disk (call xx) which shows up in my docker volumes list. When I inspect the image by $ docker volume inspect xx
I get, as expected

[
    {
        "Driver": "gce",
        "Labels": null,
        "Mountpoint": "/mnt/xx",
        "Name": "xx",
        "Options": {},
        "Scope": "local"
    }
]

However, I am trying to run an instance that runs the Google Container Optimized OS (COS) to host my docker image, and I get

docker: Error response from daemon: VolumeDriver.Mount: mkdir /rootfs/mnt/xx: read-only file system.

I've traced the problem to (I think) the fact that COS mounts /mnt as read-only. According to this,

In Container-Optimized OSs, the /mnt directory is read-only, so you must mount the local SSD at the /mnt/disks directory.

But I see no way to change the mountpoint of the volumes created by docker volume create. Would you be able to add this as an option?