Parallel overlay builds

- Closes #1018

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/app/wwctl/overlay/imprt/main.go

@@ -86,7 +86,7 @@ func CobraRunE(cmd *cobra.Command, args []string) (err error) {
 			}
 		}
 
-		return overlay.BuildSpecificOverlays(updateNodes, []string{overlayName})
+		return overlay.BuildSpecificOverlays(updateNodes, []string{overlayName}, Workers)
 	}
 
 	return nil

internal/app/wwctl/overlay/imprt/root.go

@@ -1,6 +1,8 @@
 package imprt
 
 import (
+	"runtime"
+
 	"github.com/spf13/cobra"
 	"github.com/warewulf/warewulf/internal/pkg/overlay"
 )
@@ -24,11 +26,13 @@ var (
 	}
 	NoOverlayUpdate bool
 	CreateDirs      bool
+	Workers         int
 )
 
 func init() {
 	baseCmd.PersistentFlags().BoolVarP(&NoOverlayUpdate, "noupdate", "n", false, "Don't update overlays")
 	baseCmd.PersistentFlags().BoolVarP(&CreateDirs, "parents", "p", false, "Create any necessary parent directories")
+	baseCmd.PersistentFlags().IntVar(&Workers, "workers", runtime.NumCPU(), "The number of parallel workers building overlays")
 }
 
 // GetRootCommand returns the root cobra.Command for the application.