How to start system-docker containers without internet using iPXE

We have machine that boots Rancher OS using iPXE (bare metal). It gets vmlinuz and initrd files from another local machine inside company.
We would like to use ubuntu console and we realized that without internet connection Rancher OS is not capable to boot up. In Rancher OS v0.7.1 the ubuntu console is downloaded from somewhere (docker hub, rancher server?)
The question is how can we boot up without communicating with the world.
We don’t store the state of system-docker or rancher OS, except the user docker (images are stored on local disk).
I have two ideas:

  1. It would be good if we could just put the necessary images into the build (initrd)
  2. The other solution would be be able to download the system docker images from a local private repository inside company during the boot process. So we could just put the ubuntu console there (if we would know how to tell Rancher OS to get the ubuntu console from our registry).
    Any ideas?