question on hub.docker.com: unstable versions disappearing

[teto]

In my CI, I like to pin the docker image with for instance
nixpkgs/nix-flakes@sha256:4ff08e134a935c3e94b04cbffb96680e1beed887335ee67dd1570fca8bf8f094
it looks like that image disappears every day after the CI round and is replaced with the new one
https://hub.docker.com/layers/nixpkgs/nix-flakes/latest/images/sha256-707623a2cc8d0bfba8ac7b07b20f954c55a148f481c15821d23851953b2ab91a?context=explore
(docker pull nixpkgs/nix-flakes@sha256:4ff08e134a935c3e94b04cbffb96680e1beed887335ee67dd1570fca8bf8f094 fails for instance)

1/ Is this correct ?
2/ is it possible to keep past versions up ? or should I reference for instance nixpkgs/nix-flakes:nixos-21.05 . Alternatively I could upload to hub.docker.com my copy of the image but if I can avoid it ^^