30.4 The Docker Registry

Although Ubuntu is provided with a set of tools designed to be used in place of those provided by Docker, those tools still need access to Ubuntu images for use when building containers. For this purpose, the Ubuntu team maintains a set of Ubuntu container images within the Docker Hub. The Docker Hub is an online container registry made of multiple repositories, each containing a wide range of container images available for download when building containers. The images within a repository are each assigned a repository tag (for example, 20.04, latest etc) which can be referenced when performing an image download. The following, for example, is the URL of the Ubuntu 20.04 image contained within the Docker Hub:

docker://docker.io/library/ubuntu:20.04

In addition to downloading (referred to as “pulling” in container terminology) container images from Docker and other third party hosts registries, you can also use registries to store your own...