Comment by JeanMertz

We booted a fresh GKE 1.4 cluster yesterday to migrate from our old 1.3 cluster.

Looking at the machine, it looks like this is a fork(?) of CoreOS:

  $ grep core /etc/passwd
  core:x:5000:5000::/home/core:/bin/bash

  $ cat /etc/os-release
  BUILD_ID=8820.0.0
  NAME="Container-VM Image"
  GOOGLE_CRASH_ID=Lakitu
  VERSION_ID=55
  BUG_REPORT_URL=https://crbug.com/new
  PRETTY_NAME="Google Container-VM Image"
  VERSION=55
  GOOGLE_METRICS_PRODUCT_ID=26
  HOME_URL="https://cloud.google.com/compute/docs/containers/vm-image/"
  ID=gci

  $ toolbox
  Spawning container core-debian-jessie-backports on /var/lib/toolbox/core-debian-jessie-backports.
  Press ^] three times within 1s to kill container.

If not a direct fork, it certainly uses the same base Chromium OS (which is no surprise) and uses some of the same toolsets that CoreOS uses.