HADOOP-18973. Clean yum cache after installing which in hadoop2 docker image (#6273)

Dockerfile

@@ -14,10 +14,11 @@
 # limitations under the License.
 
 FROM apache/hadoop-runner
-RUN sudo yum install -y which
 ARG HADOOP_URL=https://www.apache.org/dyn/mirrors/mirrors.cgi?action=download&filename=hadoop/common/hadoop-2.10.2/hadoop-2.10.2.tar.gz
 WORKDIR /opt
 RUN sudo rm -rf /opt/hadoop && curl -LSs -o hadoop.tar.gz $HADOOP_URL && tar zxf hadoop.tar.gz && rm hadoop.tar.gz && mv hadoop* hadoop && rm -rf /opt/hadoop/share/doc
+RUN sudo yum install -y which && \
+      sudo yum clean all
 WORKDIR /opt/hadoop
 ADD log4j.properties /opt/hadoop/etc/hadoop/log4j.properties
 RUN sudo chown -R hadoop:users /opt/hadoop/etc/hadoop/*