Unreal Engine Build Graph Copy issue

Since there're not too much useful info in Unreal Engine documentation, I have to dig this up by myself

ERROR: Source file 'C:\workspace\Binaries\Win64\UnrealEditor.target' is not under 'C:\workspace\UnrealEngine'
       while executing task <Copy Files="#InternalBinaries" From="C:\workspace\UnrealEngine" To="C:\workspace\UnrealEngine\LocalBuilds\ArchiveForUGS\Staging" Overwrite="True" ErrorIfNotFound="False" />
       at Engine/Build/Graph/Examples/CustomBuildEditorAndTools.xml(151)
       (see C:\workspace\UnrealEngine\Engine\Programs\AutomationTool\Saved\Logs\Log.txt for full exception trace)

So this is the line causing the error

<Tag Files="#OutputFiles" Except=".../Intermediate/..." With="#ArchiveFiles"/>
<Tag Files="#ArchiveFiles" Except="*.pdb" With="#ArchiveBinaries"/>
<Copy Files="#ArchiveBinaries" From="$(RootDir)" To="$(ArchiveStagingDir)"/>

The #ArchiveBinaries are simply all the filtered files output from the compiling. $(RootDir) is the engine folder (C:\workspace\UnrealEngine) and $(ArchiveStagingDir) is some target folder that doesn't matter. The copy should remain the relative strcutre here, saying C:\workspace\path\to\file should be copied to $(ArchiveStagingDir)\path\to\file

And the error is basically saying that some files is not under the RootDir and can't be copied with relative structure remaining, which makes sense, because you can't have something like $(ArchiveStagingDir)\..\path\to\file.

And the reason why we get files not under the RootDir is simple, because we don't put our game folder as a subfolder of Unreal Engine source. Once the reason is clear, the solution is simple and straight forward:

<Option Name="MyRootDir" DefaultValue="$(RootDir)\..\" Description ="The root dirctory contain both Engine and Game folders"/>
<Copy Files="#ArchiveBinaries" From="$(MyRootDir)" To="$(ArchiveStagingDir)"/>

What we learn today, Unreal want you to put your game under Engine folder if you use the source code, but you can choose to not listen to them :)