Here is my workaround to manually update images from ghcr.io from Container Manager UI only. Enabling SSH/using CLI or registering ghcr.io. is not necessary.

• in DSM, open Container Manager - Project and select your project
• from "Action" button select "Clean"
• Switch to "Image" section and delete the images used by your project
• Go back to Project section, select your project and click "Build" (donwloads up-to-date images and starts the project)

I realize this does not answer the original question (registering the ghcr.io repo), but may help with the underlying problem - updating the projects, without the need for ssh/CLI.