I had this because I inadvertantly remove the AS tag from my first image:
ex:
FROM mcr.microsoft.com/windows/servercore:1607-KB4546850-amd64
...
.. etc ...
...
FROM mcr.microsoft.com/windows/servercore:1607-KB4546850-amd64
COPY --from=installer ["/dotnet", "/Program Files/dotnet"]
... etc ...
should have been:
FROM mcr.microsoft.com/windows/servercore:1607-KB4546850-amd64 AS installer
...
.. etc ...
...
FROM mcr.microsoft.com/windows/servercore:1607-KB4546850-amd64
COPY --from=installer ["/dotnet", "/Program Files/dotnet"]
... etc ...