improve docset.yml locating routine

src/Elastic.Markdown/BuildContext.cs

@@ -42,9 +42,10 @@ public BuildContext(IFileSystem readFileSystem, IFileSystem writeFileSystem, str
 		ReadFileSystem = readFileSystem;
 		WriteFileSystem = writeFileSystem;
 
-		var docsFolder = FindDocsFolderFromRoot();
-
-		SourcePath = !string.IsNullOrWhiteSpace(source) ? ReadFileSystem.DirectoryInfo.New(source) : docsFolder;
+		var rootFolder = !string.IsNullOrWhiteSpace(source)
+			? ReadFileSystem.DirectoryInfo.New(source)
+			: ReadFileSystem.DirectoryInfo.New(Path.Combine(Paths.Root.FullName, "docs"));
+		SourcePath = FindDocsFolderFromRoot(rootFolder);
 
 		OutputPath = !string.IsNullOrWhiteSpace(output)
 			? WriteFileSystem.DirectoryInfo.New(output)
@@ -59,12 +60,14 @@ public BuildContext(IFileSystem readFileSystem, IFileSystem writeFileSystem, str
 		Git = GitConfiguration.Create(ReadFileSystem);
 	}
 
-	private IDirectoryInfo FindDocsFolderFromRoot()
+	private IDirectoryInfo FindDocsFolderFromRoot(IDirectoryInfo rootPath)
 	{
-		var defaultDocsFolder = ReadFileSystem.DirectoryInfo.New(Path.Combine(Paths.Root.FullName, "docs"));
-		var root = ReadFileSystem.DirectoryInfo.New(Paths.Root.FullName);
-		var docsFolder = root.EnumerateFiles("docset.yml", SearchOption.AllDirectories).FirstOrDefault();
-		return docsFolder?.Directory ?? defaultDocsFolder;
+		if (rootPath.Exists &&
+		    ReadFileSystem.File.Exists(Path.Combine(rootPath.FullName, "docset.yml")))
+			return rootPath;
+
+		var docsFolder = rootPath.EnumerateFiles("docset.yml", SearchOption.AllDirectories).FirstOrDefault();
+		return docsFolder?.Directory ?? throw new Exception($"Can not locate docset.yml file in '{rootPath}'");
 	}
 
 }