Update dockerfile to use .NET core 2.0

Author: danielunderwood

Dockerfile

@@ -1,4 +1,4 @@
-FROM microsoft/dotnet:1.1-sdk AS build-env
+FROM microsoft/dotnet:2.0-sdk AS build-env
 WORKDIR /app
 
 # copy csproj and restore as distinct layers
@@ -10,7 +10,7 @@ COPY RadarrAPI/* ./
 RUN dotnet publish -c Release -o out
 
 # build runtime image
-FROM microsoft/dotnet:1.1-runtime 
+FROM microsoft/dotnet:2.0-runtime
 WORKDIR /app
 COPY --from=build-env /app/out ./
 ENTRYPOINT ["dotnet", "RadarrAPI.dll"]