Updated dockerfile to use 3.1.201-alpine for build

Author: csharpfritz

Fritz.StreamTools/Dockerfile

@@ -2,7 +2,7 @@ FROM mcr.microsoft.com/dotnet/core/aspnet:3.1 AS base
 WORKDIR /app
 EXPOSE 80
 
-FROM mcr.microsoft.com/dotnet/core/sdk:3.1 AS build
+FROM mcr.microsoft.com/dotnet/core/sdk:3.1.201-alpine AS build
 WORKDIR /src
 COPY global.json .
 COPY Fritz.StreamTools/Fritz.StreamTools.csproj Fritz.StreamTools/