As System.Drawing only supported on Windows in NET6.0, you need to use SkiaSharp instead under NET5.0 or NET6.0 in Linux, or inside docker container. Please follow the next steps to use the NetStandard dlls to get SkiaSharp to your project manually. We will use Spire.Doc as example.

Step 1: Download the latest version of Spire.Doc Pack from this link, unzip it, and you'll get the DLL files for .NET Standarad from the "netstandard2.0" folder or install via NuGet.

PM> Install-Package Spire.Officefor.NETStandard 

If you already have this folder in your disk, go straight to step two.

Step 2: Create a .NET Standard library project in your Visual Studio.

Step 3: Add all DLL files under the "netstandard2.0" folder as dependencies in your project.

Right-click "Dependencies" – select "Add Reference" – click "Browse" – selcet all DLLs under "netstandard2.0" folder – click "Add".

Step 4: Install the other three packages in your project via the NuGet Package Manager. They are SkiaSharp, System.Text.Encoding.CodePages and System.Security.Cryptography.Xml.

Right-click "Dependencies" – select "Manage NuGet Packages" – click "Browse" – type the package name – select the package from the search results – click "Install".

Step 5: Now that you've added all the dependences successfully, you can start to write your own .NET Standard library that is capable of creating and processing Word documents.

using Spire.Doc;
using Spire.Doc.Documents;

namespace SpireDocStandard
{
    public class Class1
    {
        public void CreateWord()
        {
            //Create a document object
            Document doc = new Document();

            //Add a section
            Section section = doc.AddSection();

            //Add a paragrah
            Paragraph paragraph = section.AddParagraph();

            //Append text to the paragraph
            paragraph.AppendText("Hello World");

            //Save to file
            doc.SaveToFile("Output.docx", FileFormat.Docx2013);
        }  
    }
}