Portus is described as 'open source authorization service and user interface for the next generation Docker Registry'. There are five alternatives to Portus for a variety of platforms, including Online / Web-based, Self-Hosted solutions, SaaS, Docker and Kubernetes. The best alternative is Dedicated Container Registry. It's not free, so if you're looking for a free alternative, you could try Docker Hub or VMware Harbor. Other great apps like Portus are quay.io (Paid) and Google Container Registry (Freemium).
No frills and best-in-class Harbor based Container Management Solution for individuals, teams and software vendor who want distribute container images instead of ZIP files. Free and unlimited private repositories for individuals.
Docker Hub is a cloud-based registry service which allows you to link to code repositories, build your images and test them, stores manually pushed images, and links to Docker Cloud so you can deploy images to your hosts.