message: add BASE_IMAGE to Dockerfile.train
diff: # See the License for the specific language governing permissions and
  # limitations under the License.
  +ARG BASE_IMAGE=tensorflow
  ARG IMAGE_VER=21.09-tf2-py3
  -FROM nvcr.io/nvidia/tensorflow:${IMAGE_VER} AS devel
  +FROM nvcr.io/nvidia/${BASE_IMAGE}:${IMAGE_VER} AS devel
  ARG RELEASE=false
  ARG SM="60;61;70;75;80"
  