Updated instructions to use latest SDK, cleaned up unnecessary import… (#883)

* Updated instructions to use latest SDK, cleaned up unnecessary imports and added a verification step

* Further simplified the sample

* Revert "Further simplified the sample"

This reverts commit 647bd917d2.

serving/samples/helloworld-csharp/Dockerfile

@@ -1,6 +1,6 @@
 # Use Microsoft's official .NET image.
 # https://hub.docker.com/r/microsoft/dotnet
-FROM microsoft/dotnet:2.1-sdk
+FROM microsoft/dotnet:2.2-sdk
 
 # Install production dependencies.
 # Copy csproj and restore as distinct layers.

serving/samples/helloworld-csharp/Program.cs

@@ -1,12 +1,6 @@
 using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Linq;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore;
 using Microsoft.AspNetCore.Hosting;
-using Microsoft.Extensions.Configuration;
-using Microsoft.Extensions.Logging;
 
 namespace helloworld_csharp
 {

serving/samples/helloworld-csharp/README.md

@@ -1,6 +1,6 @@
 # Hello World - .NET Core sample
 
-A simple web app written in C# using .NET Core 2.1 that you can use for testing.
+A simple web app written in C# using .NET Core 2.2 that you can use for testing.
 It reads in an env variable `TARGET` and prints "Hello \${TARGET}!". If TARGET
 is not specified, it will use "World" as the TARGET.
 
@@ -11,7 +11,7 @@ is not specified, it will use "World" as the TARGET.
   if you need to create one.
 - [Docker](https://www.docker.com) installed and running on your local machine,
   and a Docker Hub account configured (we'll use it for a container registry).
-- You have installed [.NET Core SDK 2.1](https://www.microsoft.com/net/core).
+- You have installed [.NET Core SDK 2.2](https://www.microsoft.com/net/core).
 
 ## Recreating the sample code
 
@@ -19,6 +19,13 @@ While you can clone all of the code from this directory, hello world apps are
 generally more useful if you build them step-by-step. The following instructions
 recreate the source files from this folder.
 
+1. First, make sure you have [.NET Core SDK 2.2](https://www.microsoft.com/net/core) installed:
+
+   ```shell
+   dotnet --version
+   2.2.102
+   ```
+
 1. From the console, create a new empty web project using the dotnet command:
 
    ```shell
@@ -58,7 +65,7 @@ recreate the source files from this folder.
    ```docker
    # Use Microsoft's official .NET image.
    # https://hub.docker.com/r/microsoft/dotnet
-   FROM microsoft/dotnet:2.1-sdk
+   FROM microsoft/dotnet:2.2-sdk
 
    # Install production dependencies.
    # Copy csproj and restore as distinct layers.

serving/samples/helloworld-csharp/Startup.cs

@@ -1,7 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;