openshift-os-master: change from to inputs (#62376)

Using `from` will only replace the last `FROM` in the image. But we
want the flexibility to switch to a multi-stage build in the future if
we want. Let's switch to using `inputs` instead.

Author: jlebon

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

@@ -32,7 +32,10 @@ images:
   - name: OPENSHIFT_CI
     value: "1"
   dockerfile_path: Containerfile
-  from: rhel-coreos-base
+  inputs:
+    rhel-coreos-base:
+      as:
+      - quay.io/openshift-release-dev/ocp-v4.0-art-dev:c9s-coreos
   to: rhel-coreos
 - dockerfile_path: extensions/Dockerfile
   inputs: