List of free private docker registries

canister
UI: canister.io
Limit: 20

Canister offers up to 20 free private repositories. The interface works ok, a bit buggy sometimes and gives a good user experience.

docker hub
UI: hub.docker.com
Limit: 1

Original, 1 free private repo, from the source. A good choice if you only need 1 repo.

Github packages
UI: github.com/features/packages
Limit:500MB

Github introduced packages, if you only use pretty slim images, then Github packages might be a choice.

TreeScale
UI: treescale.com
Limit: 1GB

Polished UI, works fine if you can limit you repositories to 1GB in size.

JFrog
UI: jfrog.com
Limit: 2GB

Another alternative from JFrog, UI is sometimes unintuitive.