Make sure you switch the SHELL first:
SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]
RUN [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
RUN Invoke-WebRequest -UseBasicParsing -Uri 'https://github.com/git-for-windows/git/releases/download/v2.25.1.windows.1/Git-2.25.1-64-bit.exe' -OutFile 'outfile.exe'