Add Docker update info for 2.2 (dotnet#9827)

Author: scottaddie

aspnetcore/migration/21-to-22.md

@@ -78,6 +78,17 @@ services.AddMvc()
         .SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
 ```
 
+## Update Docker images
+
+The following table shows the Docker image tag changes:
+
+|2.1                                       |2.2                                       |
+|------------------------------------------|------------------------------------------|
+|`microsoft/dotnet:2.1-aspnetcore-runtime` |`microsoft/dotnet:2.2-aspnetcore-runtime` |
+|`microsoft/dotnet:2.1-sdk`                |`microsoft/dotnet:2.2-sdk`                |
+
+Change the `FROM` lines in your *Dockerfile* to use the new image tags in the preceding table's 2.2 column.
+
 ## Additional resources
 
 * <xref:mvc/compatibility-version>