Variable handling during windows docker build

Question

How can i define variables in the Dockerfile for a windows image so they are accessible in subsequent RUN statements?

My test Dockerfile looks like this

FROM microsoft/windowsservercore

SHELL ["powershell", "-Command"]

ARG viaArg=value
ENV viaEnv="""value"""
RUN $viaShell=\"value\"
RUN echo """viaArg: $env:viaArg"""
RUN echo """viaEnv: $env:viaEnv"""
RUN echo """viaShell: $myusername"""
RUN echo """viaShell+env: $env:viaShell"""

when building the image via

PS > docker build -t myImage .

only viaArg and viaEnv show (a) value.

The first example here puts multiple commands in one line like

RUN $someVar=\"2.60.3\" ; echo $someVar

but surely there must be a different way.

Is using the $env namespace the only possible way?

How does setx take part in all of this?

Note: im not trying to pass variables from commandline, so ARG may already be used in a wrong manner. And i dont want to set the variables permanently, they should only exist during the build run, so i dont need them in the registry.

Used versions are

PS > docker -v
Docker version 17.12.0-ce, build c97c6d6
PS > [System.Environment]::OSVersion.Version    
Major  Minor  Build  Revision
-----  -----  -----  --------
10     0      14393  0
PS > (Get-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion" -Name ReleaseId).ReleaseId
1607