If you're using Vagrant, you'll have to manually shut off the VM via `vagrant halt`.

If you're just trying to get started with Docker, Boot2Docker (https://github.com/boot2docker/boot2docker) is probably the way to go.

If you're looking for an Ubuntu-based Vagrant box, you can try out my Vagrant box (http://ferry.opencore.io/en/latest/install.html#os-x). It's based off of 14.04. Be warned though, my box is very large (~4GB) as it contains many pre-baked images (Hadoop, Cassandra, etc.).