新增learn-kubernetes（https://github.com/yyong-brs/learn-kubernetes）相关文件

learn/learn-kubernetes-master/kiamol/ch20/docker-images/user-controller/Dockerfile

@@ -0,0 +1,16 @@
+FROM mcr.microsoft.com/dotnet/core/sdk:3.1.301-alpine AS builder
+
+WORKDIR /src
+COPY src/UserController.csproj .
+RUN dotnet restore
+
+COPY src/ .
+RUN dotnet publish -c Release -o /out UserController.csproj
+
+# app image
+FROM mcr.microsoft.com/dotnet/core/runtime:3.1.5-alpine
+
+WORKDIR /app
+ENTRYPOINT ["dotnet", "UserController.dll"]
+
+COPY --from=builder /out/ .