Support '!=' operator in filters and record its usage

cli/command/system/prune.go

@@ -144,9 +144,14 @@ func confirmationMessage(dockerCli command.Cli, options pruneOptions) string {
 	if pruneFilters.Len() > 0 {
 		// TODO remove fixed list of filters, and print all filters instead,
 		// because the list of filters that is supported by the engine may evolve over time.
+
 		for _, name := range []string{"label", "label!", "until"} {
-			for _, v := range pruneFilters.Get(name) {
-				filters = append(filters, name+"="+v)
+			for v, isEqualOp := range options.filter.Value().GetPair(name) {
+				op := "="
+				if !isEqualOp {
+					op = "!="
+				}
+				filters = append(filters, name+op+v)
 			}
 		}
 		sort.Slice(filters, func(i, j int) bool {

opts/opts.go

@@ -345,15 +345,25 @@ func (o *FilterOpt) Set(value string) error {
 	if value == "" {
 		return nil
 	}
-	if !strings.Contains(value, "=") {
-		return errors.New("bad format of filter (expected name=value)")
+
+	var isEqualOp bool
+	// isEqualOp determines the comparison type:
+	// true  => use ="
+	// false => use "!="
+	var sep string
+	if strings.Contains(value, "!=") {
+		isEqualOp, sep = false, "!="
+	} else if strings.Contains(value, "=") {
+		isEqualOp, sep = true, "="
+	} else {
+		return errors.New("bad format of filter (expected name=value or name!=value)")
 	}
-	name, val, _ := strings.Cut(value, "=")
+	name, val, _ := strings.Cut(value, sep)
 
 	// TODO(thaJeztah): these options should not be case-insensitive.
 	name = strings.ToLower(strings.TrimSpace(name))
 	val = strings.TrimSpace(val)
-	o.filter.Add(name, val)
+	o.filter.Add(name, val, isEqualOp)
 	return nil
 }