chore: fix kubeflowkatib/mxnet-mnist image

Author: tenzen-y

examples/v1beta1/trial-images/mxnet-mnist/Dockerfile

@@ -1,8 +1,9 @@
-FROM mxnet/python:latest_cpu_native_py3
+FROM python:3.9
 
 ADD examples/v1beta1/trial-images/mxnet-mnist /opt/mxnet-mnist
 WORKDIR /opt/mxnet-mnist
 
+RUN pip install mxnet==1.9.0
 RUN chgrp -R 0 /opt/mxnet-mnist \
   && chmod -R g+rwX /opt/mxnet-mnist