Reading 'Kubernetes in Action' by Marco Luksa -- mount point woes

Hello,
I am reading Kubernetes in Action (v1), by Marco Luksa.
In 6.4.1, he creates a mongodb volume in a GCE context.
But I cannot create the /data/db mount point – can I use /mnt/disks/mongodb ?

I can ssh to the node scheduled for the pod I created, and there:

$ sudo lsblk | egrep ^sdb
sdb 8:16 0 10G 0 disk
$ sudo mkdir -p /data/db
mkdir: cannot create directory ‘/data’: Read-only file system
$ sudo mkdir -p /mnt/disks/mongodb
$ sudo mount -o discard,defaults /dev/sdb /mnt/disks/mongodb
$ sudo chmod a+w /mnt/disks/mongodb

I did set /mnt/disks/mongodb (instead of /data/db) as mountPoint in the yaml file I used to create my cluster.
But the container would not start:

gcloud> k get po mongodb
NAME READY STATUS RESTARTS AGE
mongodb 0/1 ContainerCreating 0 25m
gcloud> k logs --since=60s --timestamps=true mongodb
Error from server (BadRequest): container “mongodb” in pod “mongodb” is waiting to start: ContainerCreating

I guess mongodb attempts to write to /data/db
How can I drive it to use /mnt/disks/mongodb instead?

Thanks
Marc

Just to complete my yesterday post, here is a link to the pod definition (yaml), specifying the volumen type: gcePersistentDisk, and the original mount point, which I failed to create, hence replaced with the one I could create on the node.
In my code snippet, gcloud> was my prompt, and k an alias to kubectl.