Docker images are stored in registries. There are two different kinds of Docker registries: public and private. Public registries, such as Docker hub, hold images that can build hundreds of software systems including:
● Apache or Nginx web servers
● MySQL, Cassandra, or MongoDB database servers
● Asterisk or Elasticsearch software packages
Explore the public Docker images on the hub via the search command on any page at https://hub.docker.com. Private registries hold Docker images that companies want to have complete control over. Companies and other organizations can host their own registries. In addition, Docker can host private registries for organizations that need only a few images stored, or don’t want to maintain the registry themselves. (Note: this latter arrangement is very similar to how Github.com, the software control site, manages its hub.)
The Dockerfile shown in the previous section creates an image that causes an Apache server to run on Ubuntu, and it starts by getting an Ubuntu image. If that image is not on the local machine, then the Docker software asks https://hub.docker.com (or a specified registry) to provide the Ubuntu image. Thus, another way to think of a registry is a place where images wait to be invoked.
The newly created image (the end result of the Dockerfile in the previous section) might be stored in a repository on https://hub.docker.com as well. If the repository is called “fake-name-company”, then the image might be stored as fake-name-company/apache.