You are creating a container for an ASP.

Box 1: microsoft.com/dotnet/sdk:2.3

The first group of lines declares from which base image we will use to build our container ontop of. If the local system does not have this image already, then docker will automatically tryand fetch it. The mcr.microsoft.com/dotnet/core/sdk:2.1 comes packaged with the .NET core 2.1 SDK installed, so it's up to the task of building ASP .NET core projects targeting version 2.1

Box 2: dotnet restore

The next instruction changes the working directory in our container to be /app, so all commands following this one execute under this context.

COPY *.csproj ./

RUN dotnet restore

Box 3: microsoft.com/dotnet/2.2-aspnetcore-runtime

When building container images, it's good practice to include only the production payload and its dependencies in the container image. We don't want the .NET core SDK included in our final image because we only need the .NET core runtime, so the dockerfile is written to use a temporary container that is packaged with the SDK called build-env to build the app.