Update ASP.NET Core 5.0 migration guide for Preview 4 (#18489)

* Update ASP.NET Core 5.0 migration guide for Preview 4

* minor edit

* Add link to GitHub issue

Author: scottaddie

aspnetcore/migration/31-to-50.md

@@ -4,7 +4,7 @@ author: scottaddie
 description: Learn how to migrate an ASP.NET Core 3.1 project to ASP.NET Core 5.0.
 ms.author: scaddie
 ms.custom: mvc
-ms.date: 04/28/2020
+ms.date: 05/26/2020
 no-loc: [Blazor, "Identity", "Let's Encrypt", Razor, SignalR]
 uid: migration/31-to-50
 ---
@@ -41,7 +41,7 @@ If you rely upon a [global.json](/dotnet/core/tools/global-json) file to target
 {
   "sdk": {
 -    "version": "3.1.200"
-+    "version": "5.0.100-preview.3.20216.6"
++    "version": "5.0.100-preview.4.20258.7"
   }
 }
 ```
@@ -69,19 +69,22 @@ In the project file, update each `Microsoft.AspNetCore.*` and `Microsoft.Extensi
 <ItemGroup>
 -    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="3.1.2" />
 -    <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="3.1.2" />
-+    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="5.0.0-preview.3.20215.14" />
-+    <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="5.0.0-preview.3.20215.2" />
++    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="5.0.0-preview.4.20257.10" />
++    <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="5.0.0-preview.4.20251.6" />
 </ItemGroup>
 ```
 
 ## Update Docker images
 
-For apps using Docker, use a base image that includes the ASP.NET Core 5.0 runtime. For example:
+For apps using Docker, update your *Dockerfile* `FROM` statements and scripts. Use a base image that includes the ASP.NET Core 5.0 runtime. Consider the following `docker pull` command difference between ASP.NET Core 3.1 and 5.0:
 
-```bash
-docker pull mcr.microsoft.com/dotnet/core/aspnet:5.0
+```diff
+- docker pull mcr.microsoft.com/dotnet/core/aspnet:3.1
++ docker pull mcr.microsoft.com/dotnet/aspnet:5.0
 ```
 
+As part of the move to ".NET" as the product name, the Docker images moved from the `mcr.microsoft.com/dotnet/core` repositories to `mcr.microsoft.com/dotnet`. For more information, see [dotnet/dotnet-docker#1939](https://github.com/dotnet/dotnet-docker/issues/1939).
+
 ## Review breaking changes
 
 For breaking changes from .NET Core 3.1 to .NET 5.0, see [Breaking changes for migration from version 3.1 to 5.0](/dotnet/core/compatibility/3.1-5.0). ASP.NET Core and Entity Framework Core are also included in the list.