list/tree: Hide `Used` column if no truthy value

Don't show the `Used` column in the image tree view when no image is
actually used and the column would be empty.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli/command/image/tree.go

@@ -3,6 +3,7 @@ package image
 import (
 	"context"
 	"fmt"
+	"slices"
 	"sort"
 	"strings"
 	"unicode/utf8"
@@ -25,6 +26,9 @@ type treeOptions struct {
 type treeView struct {
 	images []topImage
 
+	// showUsed indicates whether the "Used" column should be shown.
+	showUsed bool
+
 	// imageSpacing indicates whether there should be extra spacing between images.
 	imageSpacing bool
 }
@@ -71,6 +75,9 @@ func runTree(ctx context.Context, dockerCLI command.Cli, opts treeOptions) error
 			if sub.Details.Used {
 				// Mark top-level parent image as used if any of its subimages are used.
 				details.Used = true
+
+				// Show the Used column only if there will be at least one non-zero value.
+				view.showUsed = true
 			}
 
 			totalContent += im.Size.Content
@@ -192,6 +199,12 @@ func printImageTree(dockerCLI command.Cli, view treeView) error {
 		},
 	}
 
+	if !view.showUsed {
+		columns = slices.DeleteFunc(columns, func(c imgColumn) bool {
+			return c.Title == "Used"
+		})
+	}
+
 	nameWidth := int(width)
 	for idx, h := range columns {
 		if h.Width == 0 {