Docker's eventually going away and distros will replace it with a home grown alternative (or systemd). I predict that in 10 years distros will finally move toward a container native model (different from what we have now, but still based on containerization) and host their own registries for officially packaged containers. The same people that mirror their packages will host the registries. You'll add trusted registries the same way you add trusted package repos. In theory, this might actually reduce their overall bandwidth use.