I am looking at the docker images hosted on Docker Hub but they appear to have not been updated for a few months. In particular, the 2.x series hasn’t made it on there yet.
Who is responsible for this images? Would it be easy to push an updated version? Should updating these images be part of the release process?
I’m not sure who is responsible for that particular image/account, but there is another Nix image that appears to be much more active:
The official nix docker image is published from building https://github.com/NixOS/nix/blob/master/misc/docker/Dockerfile . The process is manual which explains why it happens infrequently.
Getting a more recent release and getting rid of the alpine dependency was the main motivation behind creating nix-community/docker-nix.
There is also GitHub - LnL7/nix-docker: Docker images for the Nix package manager which is god maintained. But ideally the process of updating could be autotomized.
LnL7 is a god? 
So that last image is the last approach where the docker image is generated from nix code instead of using a Dockerfile. I think we have all the combinations now!
Updated Workgroup:Container - NixOS Wiki with the list
It probably wouldn’t be that much work to automate the updates. Everything is generated by nix except for the nixpkgs source expression.
One other angle would be to make nixpkgs channel-based releases. Every time the channel advances, push another image on the same tag (eg: nixos/nixpkgs:18.03). That would make it quite easy to get fresh images and avoid a secondary download step to get the updated channel. It could even be made part of the nixpkgs release.nix.
I took over docker maintenance back then just to keep things moving and that’s where my ambition still is today. If someone wants to take this further and invest more time, let’s talk 
The official nixos/nix image is now managed in a different repo. Thanks @peti !
https://github.com/NixOS/nix/commit/81261ca5381eb503b27a529bb9d35383e79f98b8
Is it Docker official docker image?
I can not find link to it from nixos.org . What about making link to official docker hub image e.g. in Download section Download Nix / NixOS | Nix & NixOS ?