A registry (repository) provides storage and versioning for Docker images. Installing a (private) Docker registry can be useful if you want to securely host your own algorithms.
Docker provides a registry as a turn-key solution on Docker Hub. Instructions for setting it up can be found here: https://hub.docker.com/_/registry.
Harbor (https://goharbor.io) is another option for running a registry. In addition it also provides things like access control.