hack/build: Pin to RHCOS 47.212 and quay.io/openshift-release-dev/ocp-release:4.0.0-6

That's the latest RHCOS release:

  $ curl -s https://releases-rhcos.svc.ci.openshift.org/storage/releases/maipo/builds.json | jq '{latest: .builds[0], timestamp}'
  {
    "latest": "47.212",
    "timestamp": "2018-12-14T23:43:06Z"
  }

And Clayton just pushed 4.0.0-0.alpha-2018-12-13-230701 to
quay.io/openshift-release-dev/ocp-release:4.0.0-6.

Renaming OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE gets us CI testing
of the pinned release despite openshift/release@60007df2 (Use
RELEASE_IMAGE_LATEST for CVO payload, 2018-10-03,
openshift/release#1793).

I'd initially expected to export the pinning environment variables in
release.sh, but I've put them in build.sh here because our continuous
integration tests use build.sh directly and don't go through
release.sh.

Author: wking

hack/build.sh

@@ -2,6 +2,9 @@
 
 set -ex
 
+RELEASE_IMAGE="${RELEASE_IMAGE:-quay.io/openshift-release-dev/ocp-release:4.0.0-6}"
+RHCOS_BUILD_NAME="${RELEASE_BUILD_NAME:-47.212}"
+
 # shellcheck disable=SC2068
 version() { IFS="."; printf "%03d%03d%03d\\n" $@; unset IFS;}
 

pkg/asset/ignition/bootstrap/bootstrap.go

@@ -144,7 +144,7 @@ func (a *Bootstrap) getTemplateData(installConfig *types.InstallConfig) (*bootst
 	}
 
 	releaseImage := defaultReleaseImage
-	if ri, ok := os.LookupEnv("OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE"); ok && ri != "" {
+	if ri, ok := os.LookupEnv("_OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE"); ok && ri != "" {
 		logrus.Warn("Found override for ReleaseImage. Please be warned, this is not advised")
 		releaseImage = ri
 	}