Original report here: dotnet/sdk-container-builds#192 (comment)

If the user creates a new regular file tar entry, and sets the DataStream to an unseekable stream, we should either:

• Throw immediately if the stream is unseekable in the TarEntry.cs#L251 setter, or...
• Find a way to retrieve the unseekable stream's length and position without calling Length, right before writing the data to the archive: TarHeader.Write.cs#L510-L511

cc @jozkee