The DigitalOcean one-click App uses nodesource.
I recommend updating to Node v10 (v8 is almost end of life)
curl -sL https://deb.nodesource.com/setup_10.x | sudo -E bash -
sudo apt-get install -y nodejs
ghost update --force
One you’ve updated node, you must run ghost update --force in the ghost directory, this will force Ghost to update its dependencies to match Node v10, otherwise you’ll get more errors.