Docker registries are storage hosts for your docker containers. In CI/CD its is common to build, test and then push to a registry for use later. These are a few common registries you can use. Some of which support private, private and both.
There are a few which provide registries as part of their cloud SaaS offering
Some registries also support other forms of data and binaries not only docker containers, it can be a proxy for your own nuget packages, npm packages where it syncs with other registries and manages it own
And finally of course, you can setup your own registry, vultr cloud host has a nice article about this
Did you see any missing? post in a reply via comments below or ping me via twitter / linkedin @mirageglobe