Add dockertest

[vinckr]

Already have a PR ready, but I figured I would ask for your approval first 😄

Dockertest is a project developed to make Go integration tests really easy with Docker.
It is really popular on github, so I thought it might be a good addition to this list, let me know what you think.
Even Google is using it for some of their projects.

Docker is the perfect system for running unit tests as you can spin up containers in a few seconds and kill them when the test completes.
The Dockertest library provides easy to use commands for spinning up Docker containers and using them for your tests.+
Use Docker to run your Go language integration tests against third party services on Microsoft Windows, Mac OSX and Linux!

[disclaimer: i work for ory]