Add 'LABEL' command from '--label' to the last stage

This PR is tring to fix issue #36996.

Currently for multi-stage build, if `--target` specified, the `--label` option
will be ignored. The root cause is the last stage build will remove the `LABEL`
command(s) node created from the `--label` option. In order to address this issue,
we can create `LABEL` command(s) and add it/tem to the last stage.

Signed-off-by: Dennis Chen <dennis.chen@arm.com>

rewritten from github.com/moby/moby 9c238ebd55e4105ad7f7edc04231ea61bb278ae8

frontend/dockerfile/instructions/commands.go

@@ -110,17 +110,37 @@ type MaintainerCommand struct {
 	Maintainer string
 }
 
+// NewLabelCommand creates a new 'LABEL' command
+func NewLabelCommand(k string, v string, NoExp bool) *LabelCommand {
+	kvp := KeyValuePair{Key: k, Value: v}
+	c := "LABEL "
+	c += kvp.String()
+	nc := withNameAndCode{code: c, name: "label"}
+	cmd := &LabelCommand{
+		withNameAndCode: nc,
+		Labels: KeyValuePairs{
+			kvp,
+		},
+		noExpand: NoExp,
+	}
+	return cmd
+}
+
 // LabelCommand : LABEL some json data describing the image
 //
 // Sets the Label variable foo to bar,
 //
 type LabelCommand struct {
 	withNameAndCode
-	Labels KeyValuePairs // kvp slice instead of map to preserve ordering
+	Labels   KeyValuePairs // kvp slice instead of map to preserve ordering
+	noExpand bool
 }
 
 // Expand variables
 func (c *LabelCommand) Expand(expander SingleWordExpander) error {
+	if c.noExpand {
+		return nil
+	}
 	return expandKvpsInPlace(c.Labels, expander)
 }