list/tree: Capitalize column headers

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 0242a1e3c6)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

cli/command/image/tree.go

@@ -207,7 +207,7 @@ func printImageTree(dockerCLI command.Cli, images []topImage) error {
 			_, _ = fmt.Fprint(out, strings.Repeat(" ", columnSpacing))
 		}
 
-		_, _ = fmt.Fprint(out, h.Print(headerColor, h.Title))
+		_, _ = fmt.Fprint(out, h.Print(headerColor, strings.ToUpper(h.Title)))
 	}
 
 	_, _ = fmt.Fprintln(out)