The 2.1 preview Alpine images are based on 3.6 - e.g. https://github.com/dotnet/dotnet-docker/blob/master/2.1/runtime/alpine/amd64/Dockerfile. This should be changed to the latest version of Alpine - 3.7. Depending on when 2.1 ships, 3.8 may be available.
This change is dependent on the completion of https://github.com/dotnet/core-setup/issues/3817