Like many people here I pre-configure my docker images with everything including the autoinstalled contents of the [WORKDIR]/tools directory (maven, gradle, groovy, etc). This works great with the docker plugin, not so much with k8s.
My current workaround is to symlink the tools directory into the Working Directory in my ENTRYPOINT script when running in k8s (my images are also used outside of k8s)
if [ -n "$JENKINS_AGENT_WORKDIR" ] && [ "$JENKINS_AGENT_WORKDIR" != "$NORMAL_WORKDIR" ] ; then
echo "Linking preinstalled Jenkins tools into the active workdir: $JENKINS_AGENT_WORKDIR"
ln -s $NORMAL_WORKDIR/tools $JENKINS_AGENT_WORKDIR/
(My ENTRYPOINT script is a wrapper around /usr/local/bin/jenkins-slave)