Why $(SolutionDir) is undefined in dotnet CLI?

I'm trying to build my project from dotnet CLI.

I'm using dotnet build and it fails.

How to reproduct:

1. Create a simple console application using Visual Studio template

2. Add a file named Colors.txt to the solution directory

3. Add a file named Names.txt to the project directory

4. Modify csproj file to include a post-build event

   <PropertyGroup>
       <OutputType>Exe</OutputType>
       <TargetFramework>netcoreapp3.1</TargetFramework>
   </PropertyGroup>
   
   <Target Name="CopySettingsToOutput" AfterTargets="PostBuildEvent">
       <Exec Command="copy $(ProjectDir)\Names.txt $(TargetDir)\Names.txt" />
       <Exec Command="copy $(SolutionDir)\Colors.txt $(TargetDir)\Colors.txt" />
   </Target>
   

5. Now build using Visual Studio (it gets built)

6. Now build using dotnet build command and it fails. It says:

error MSB3073: The command "copy Undefined\Colors.txt C:\---path---\Colors.txt" exited with code 1.

Why $(SolutionDir) is undefined when using dotnet build?