Update INSTALL-digital-ocean.md

Why not use the docker install script? It has checks to make sure things are installed correctly. It could also be done via curl: 'curl -sL https://get.docker.io/ | sh'.

There's also https://get.docker.io/ubuntu/ as well, which has less checks.

Finally, should there be steps to set up an additional user and disable root login? Seems a little insecure.