Where can I find a Windows Server 2019 installation image to be used with Nano Server Image Builder?

I want to build Docker container on a Windows Server 2019 host. The Docker containers should be based on Nano Server 2019.
I already succeeded in downloading a Nano Server 2019 container which fits to my Windows Server 2019 host OS version 1809 OS Build 17763.557.
In a next step, i want to extend this Nano Server image with Windows Server 2019 components which I will need for my application which i want to run in the container. We do not want to use Windows Server Core because we hope that our basic container based on Nano Server will be significantly smaller than Windows Server Core.

My questions:
(1) Why there is no Nano Server Image Builder for Windows Server 2019 available?
(2) Where can i download a Windows Server 2019 ISO Image which contains a Nano .wim image (as it is required by Nano Server Image Builder)
(3) Windows Server 2019 Nano Server comes without Powershell, so how can i install Powershell into this Windows Server 2019 Nano Server container?

Hope someone can give me some directions where I can solve these problems, thanks in advance!