Show how to use ASPNETCORE_ENVIRONMENT with Migrations

[cni13]

The mentioned section misses to explain some (for me) important details you need if you want to use this approach:
The call Program.BuildWebHost is implicitly called with an ASPNETCORE_ENVIRONMENT setting of "development". That means that if you read the connection string from e.g. the appsettings inside AddDbContext, you need to have an appsettings file for this environment variable. Otherwise the root appsettings will be used, which means that you have to also copy the connection string to the database you want to use to this file. Otherwise this approach will fail, because it cant find the connection string.
Also in general the application needs to be able to start with the "development" env variable. If it for some reason does not build, you might face exceptions that you can't associate with the source of the problem.

My proposal:

• Further explain the behavior of the mentioned section, including the implicit usage of "development" as env var
• Perhaps give a short example on how to use this approach in practice

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Document Details

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• ID: 2fa64390-1232-8dd3-f170-7d5c68e29ad9
• Version Independent ID: 245bbba8-3102-4a66-15eb-650e05eb9e62
• Content: Design-time DbContext Creation - EF Core
• Content Source: entity-framework/core/miscellaneous/cli/dbcontext-creation.md
• Product: entity-framework
• GitHub Login: @bricelam
• Microsoft Alias: bricelam

[bricelam]

Note to documenter: Mention DOTNET_ENVIRONMENT too