Add .NET Core 2.1 versions

vote/dotnet/Dockerfile

@@ -0,0 +1,16 @@
+FROM microsoft/dotnet:2.1-sdk as builder
+
+WORKDIR /Vote
+COPY Vote/Vote.csproj .
+RUN dotnet restore
+
+COPY /Vote .
+RUN dotnet publish -c Release -o /out Vote.csproj
+
+# app image
+FROM microsoft/dotnet:2.1-aspnetcore-runtime
+
+WORKDIR /app
+ENTRYPOINT ["dotnet", "Vote.dll"]
+
+COPY --from=builder /out .