openshift/os: start building node image

As part of openshift/enhancements#1637, we want
to start building the node image as a layered build on top of an RHCOS
base image.

For now, don't promote this image. In the future, when we're ready to
switch CI over, it'll take the place of `rhel-coreos`.

ci-operator/config/openshift/os/openshift-os-master.yaml

@@ -1,3 +1,8 @@
+base_images:
+  rhel-coreos-base:
+    name: rhel-coreos-base
+    namespace: coreos
+    tag: "9.6"
 build_root:
   image_stream_tag:
     name: coreos-assembler
@@ -23,10 +28,17 @@ images:
     WORKDIR /tmp/cosa
   from: root
   to: build-image
+- build_args:
+  - name: OPENSHIFT_CI
+    value: "1"
+  dockerfile_path: Containerfile
+  from: rhel-coreos-base
+  to: rhel-coreos
 promotion:
   to:
   - excluded_images:
     - build-image
+    - rhel-coreos
     name: "4.19"
     namespace: ocp
 releases: