Added prerequisites for OpenShift image

Author: yuchaoran2011

Dockerfile.rh

@@ -15,6 +15,11 @@
 # limitations under the License.
 #
 
+# Build an OpenShift image.
+# Before running docker build, make sure
+# 1. Your Docker version is >= 18.09
+# 2. export DOCKER_BUILDKIT=1
+
 ARG SPARK_IMAGE=gcr.io/spark-operator/spark:v2.4.0
 
 FROM golang:1.11.2-alpine as builder