A registry is simply a place where images are stored. Already in this book, you have used a Docker Registry. In Chapter 1, Getting Started with Docker Orchestration, there was an example of starting an nginx container:
$ docker run --name webserver -p 80:80 nginx
The docker command includes the name of the image to use, in this case nginx. Docker first looks for that image locally, but if the image is not found locally, Docker connects to the default registry at the Docker Hub. If the image exists on the registry, it is downloaded and your container starts.
Think of how powerful having a registry can be. A registry makes it easy to quickly deploy any image to any node in your cluster. Without one, you have to manually build your image on each and every machine first. In my early days of using Docker, I did that and it was not fun. Each time I built an image, there was a chance that a library change would break my application. Using a registry gives you consistent...