Are there Docker Hub comparable public Docker image repositories?
With Docker following the Open Container Initiative, in the event that Docker were to disappear, people would still have access to images stored outside of Docker Hub.

While there is not a public mirror of Docker Hub, you do have access to other registries that often store the official image.

For example, if you were to pull the official Docker SQL Server image, you would target mcr.microsoft.com/mssql/server:2019.  So even though the image can be searched on in Docker Hub, the image still exists outside of Docker Hub.  The three big cloud providers (Azure, AWS, and Google) all have container registries that allow public access.  Many of the cloud registries support storing of images in regions and would allow "mirrored" architectures that are transparent to the public and disaster recovery (i.e. end users only pull from one link and the provider routes to an active available registry).