As mentioned earlier, a Docker image is a way to deliver applications. You can create a Docker image and share it with other users using a public/private registry service. A registry is a stateless, highly scalable server-side application which you can use to store and download Docker images. Docker registry is an open source project, under the permissive Apache license. Once the image is available on a Docker registry service, another user can download it by pulling the image and can use this image to create new Docker images or run containers from this image.

Docker supports several types of docker registry:

• Public registry
• Private registry

You can start a container from an image stored in a public registry. By default, the Docker daemon looks for and downloads Docker images from Docker Hub, which is a public registry provided by Docker. However, many vendors add their own public registries to the Docker configuration at installation time. For example, Red Hat has its own proven and blessed public Docker registry which you can use to pull Docker images and to build containers.

Some organization or specific teams don't want to share their custom container images with everyone for a reason. They still need a service to share Docker images, but just for internal usage. In that case, a private registry service can be useful. A private registry can be installed and configured as a service on a dedicated server or a virtual machine inside your network.

You can easily install a private Docker registry by running a Docker container from a public registry image. The private Docker registry installation process is no different from running a regular Docker container with additional options.

A Docker registry is accessed via the Docker daemon service using a Docker client. The Docker command line uses a RESTful API to request process execution from the daemon. Most of these commands are translated into HTTP requests and may be transmitted using curl.

The process of using Docker registries is shown in the following section.

A developer can create a Docker image and put it into a private or public registry. Once the image is uploaded, it can be immediately used to run containers or build other images.