The way I did it was to prepare my cloud-config.yml with the public key included on my personal machine.
Once booted from the ISO I then scp'd (you can sftp too) back to my machine and copied the cloud-config.yml file into RancherOS so that I could use it to do the install.
This issue is solved by creating an interface.
After this I can transfer my cloud-config.yml to the guest and start installation on disk.
I managed to deploy & launch container “Rancher” and now next challenges are ahead…