Provide option to prune Docker images when using uninstall.sh

Currently, uninstall.sh does not automatically prune the downloaded Elasticsearch/Kibana docker images.

These images will continue to use 2.6 GB of disk space, even if no container uses the image any longer.

We should ask the user in the uninstall.sh script whether they want to also remove the downloaded Docker images.

/cc @flobernd 