One of the things about having an ARM-based RPi cluster is a need to serve custom images. Even though there are a number of well run, cloud stored image registries, including Docker Hub and Google Container Registry, it feels like this is a homebrew style service that one should be able to host on their own. Straight Docker Distribution is surprisingly barebones.
Meanwhile, VMWare has open-sourced Harbor, an image registry which seems much more full featured:
Project Harbor is an an open source trusted cloud native registry project that stores, signs, and scans content. Harbor extends the open source Docker Distribution by adding the functionalities usually required by users such as security, identity and management. Harbor supports advanced features such as user management, access control, activity monitoring, and replication between instances. Having a registry closer to the build and run environment can also improve image transfer efficiency.