Apr 19 2021
I'm trying 20334 but didn't find any container images that match the 10.0.20334.1 tag at:
May 31 2021
Jun 04 2021
They appear to be there now. According to https://hub.docker.com/_/microsoft-windows-nanoserver-insider they went up on the 28th of April.
Edit: Ooops. I misread. It's 20344 that was posted on 28th of April. But the 20334 container image was up when I wrote my message, so presumably it had been uploaded earlier than that.