Linux is the default environment for the official .net core docker images. You can see this on Microsoft's official docker page[0]. For non-linux builds, you have to specifically ask for the nanoserver images.

ex:

microsoft/dotnet:latest is a debian based image[1]

microsoft/dotnet:core is a debian based image[2]

[0] - https://hub.docker.com/r/microsoft/dotnet/

[1] - https://github.com/dotnet/dotnet-docker/blob/master/1.0.0-pr...

[2] - https://github.com/dotnet/dotnet-docker/blob/master/1.0/debi...