Update from microsoft/* to mcr.microsoft.com/*. (docker#8738)

Update Dockerfile with following changes:
1. Switch .NET Core Images from legacy location at Docker Hub Registry (microsoft/*) to a new location at Microsoft Container Registry (mcr.microsoft.com/*).
2. Since Microsoft no longer supports the tag 'latest', I must set a specific version of '2.2' (current), which is the most recent stable release.

Author: ob1dev

engine/examples/dotnetcore.md

@@ -41,7 +41,7 @@ Windows](/docker-for-windows/). Read more on [switching containers](/docker-for-
    the `Dockerfile` to use the DLL file of your project.
 
 ```dockerfile
-FROM microsoft/dotnet:sdk AS build-env
+FROM mcr.microsoft.com/dotnet/core/sdk:2.2 AS build-env
 WORKDIR /app
 
 # Copy csproj and restore as distinct layers
@@ -53,7 +53,7 @@ COPY . ./
 RUN dotnet publish -c Release -o out
 
 # Build runtime image
-FROM microsoft/dotnet:aspnetcore-runtime
+FROM mcr.microsoft.com/dotnet/core/aspnet:2.2
 WORKDIR /app
 COPY --from=build-env /app/out .
 ENTRYPOINT ["dotnet", "aspnetapp.dll"]