docker-machine-update-hosts.sh

#!/usr/bin/env bash

list=( $(docker-machine ls | grep Running | awk '{ print $1 }') )

for i in "${list[@]}"; do
    ip=$(docker-machine ip $i)
    #remove line matching ip
    sudo sed -i '' '/'$ip'/d' /etc/hosts
    #insert ip/host on last line
    sudo sed -i -e '$a\    
    '"$ip"' '"$i"'' /etc/hosts
done

Yes, you are correct. Some additional logic could be added to search for host+ip.