Fix builder prune -a/--all flag description

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit aad9d2c958b24639ea1d6dd46e73b2fdb36f4b62)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: f9a5c1f152106a28d347b02e4c9ab506ad1bf04e
Component: cli

Author: thaJeztah

components/cli/cli/command/builder/prune.go

@@ -44,7 +44,7 @@ func NewPruneCommand(dockerCli command.Cli) *cobra.Command {
 
 	flags := cmd.Flags()
 	flags.BoolVarP(&options.force, "force", "f", false, "Do not prompt for confirmation")
-	flags.BoolVarP(&options.all, "all", "a", false, "Remove all unused images, not just dangling ones")
+	flags.BoolVarP(&options.all, "all", "a", false, "Remove all unused build cache, not just dangling ones")
 	flags.Var(&options.filter, "filter", "Provide filter values (e.g. 'until=24h')")
 	flags.Var(&options.keepStorage, "keep-storage", "Amount of disk space to keep for cache")