serving/helloworld-csharp: respect the port env var (#462)

Author: grayside

serving/samples/helloworld-csharp/Program.cs

@@ -17,8 +17,13 @@ public static void Main(string[] args)
             CreateWebHostBuilder(args).Build().Run();
         }
 
-        public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
-            WebHost.CreateDefaultBuilder(args)
-                .UseStartup<Startup>().UseUrls("http://0.0.0.0:8080");
+        public static IWebHostBuilder CreateWebHostBuilder(string[] args)
+        {
+            string port = Environment.GetEnvironmentVariable("PORT") ?? "8080";
+            string url = String.Concat("http://0.0.0.0:", port);
+
+            return WebHost.CreateDefaultBuilder(args)
+                .UseStartup<Startup>().UseUrls(url);
+        }
     }
 }

serving/samples/helloworld-csharp/README.md

@@ -26,12 +26,17 @@ recreate the source files from this folder.
     ```
 
 1. Update the `CreateWebHostBuilder` definition in `Program.cs` by adding
-   `.UseUrls("http://0.0.0.0:8080")` to define the serving port:
+   `.UseUrls()` to define the serving port:
 
     ```csharp
-    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
-        WebHost.CreateDefaultBuilder(args)
-            .UseStartup<Startup>().UseUrls("http://0.0.0.0:8080");
+    public static IWebHostBuilder CreateWebHostBuilder(string[] args)
+    {
+        string port = Environment.GetEnvironmentVariable("PORT") ?? "8080";
+        string url = String.Concat("http://0.0.0.0:", port);
+
+        return WebHost.CreateDefaultBuilder(args)
+            .UseStartup<Startup>().UseUrls(url);
+    }
     ```
 
 1. Update the `app.Run(...)` statement in `Startup.cs` to read and return the