When running ASP.NET Core apps, the WebHostBuilder will automatically attempt to determine which environment it is running in. By convention, this will be one of Development, Staging or Production, but you can set it to any string value you like.

This link will give you much information, but if you use IIS as proxy of ASP.NET CORE, you need to set the environment in web.config

<?xml version="1.0" encoding="utf-8"?>
      <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
    <aspNetCore processPath="dotnet" arguments=".\Host.dll" stdoutLogEnabled="true" stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false">
        <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />

IOW, you need to add the following content to web.config

        <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />

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