Explain the Dockerfile distinct layers approach (#18274)

Author: serpent5

aspnetcore/host-and-deploy/docker/building-net-docker-images.md

@@ -4,7 +4,7 @@ author: rick-anderson
 description: Learn how to use the published .NET Core Docker images from the Docker Registry. Pull images and build your own images.
 ms.author: riande
 ms.custom: mvc
-ms.date: 01/15/2020
+ms.date: 05/12/2020
 no-loc: [Blazor, "Identity", "Let's Encrypt", Razor, SignalR]
 uid: host-and-deploy/docker/building-net-docker-images
 ---
@@ -199,7 +199,6 @@ COPY aspnetapp/. ./aspnetapp/
 WORKDIR /app/aspnetapp
 RUN dotnet publish -c Release -o out
 
-
 FROM mcr.microsoft.com/dotnet/core/aspnet:2.2 AS runtime
 WORKDIR /app
 COPY --from=build /app/aspnetapp/out ./
@@ -235,21 +234,15 @@ COPY aspnetapp/. ./aspnetapp/
 WORKDIR /app/aspnetapp
 RUN dotnet publish -c Release -o out
 
-
 FROM mcr.microsoft.com/dotnet/core/aspnet:3.0 AS runtime
 WORKDIR /app
 COPY --from=build /app/aspnetapp/out ./
 ENTRYPOINT ["dotnet", "aspnetapp.dll"]
 ```
 
-::: moniker-end
+As noted in the preceding Dockerfile, the `*.csproj` files are copied and restored as distinct *layers*. When the `docker build` command builds an image, it uses a built-in cache. If the `*.csproj` files haven't changed since the `docker build` command last ran, the `dotnet restore` command doesn't need to run again. Instead, the built-in cache for the corresponding `dotnet restore` layer is reused. For more information, see [Best practices for writing Dockerfiles](https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#leverage-build-cache).
 
-```dockerfile
-FROM mcr.microsoft.com/dotnet/core/aspnet:3.0 AS runtime
-WORKDIR /app
-COPY published/aspnetapp.dll ./
-ENTRYPOINT ["dotnet", "aspnetapp.dll"]
-```
+::: moniker-end
 
 ## Additional resources