Explain a littble bit on how to prepare Dockerfile before building kube-build image

Author: zzxwill

build/README.md

@@ -34,7 +34,7 @@ The following scripts are found in the [`build/`](.) directory. Note that all sc
 
 ## Basic Flow
 
-The scripts directly under [`build/`](.) are used to build and test.  They will ensure that the `kube-build` Docker image is built (based on [`build/build-image/Dockerfile`](build-image/Dockerfile)) and then execute the appropriate command in that container.  These scripts will both ensure that the right data is cached from run to run for incremental builds and will copy the results back out of the container.
+The scripts directly under [`build/`](.) are used to build and test.  They will ensure that the `kube-build` Docker image is built (based on [`build/build-image/Dockerfile`](build-image/Dockerfile) and after base image's `KUBE_BUILD_IMAGE_CROSS_TAG` from Dockerfile is repalced with one of those actual tags of the base image, like `v1.13.9-2`)) and then execute the appropriate command in that container.  These scripts will both ensure that the right data is cached from run to run for incremental builds and will copy the results back out of the container.
 
 The `kube-build` container image is built by first creating a "context" directory in `_output/images/build-image`.  It is done there instead of at the root of the Kubernetes repo to minimize the amount of data we need to package up when building the image.