func newLabelDirective(base *baseDirective, state *parsingState) (Directive, error) {
	if err := base.replaceVarsCurrStage(state); err != nil {
		return nil, err
	}
	labels, err := parseKeyVals(base.Args)
	if err != nil {
		return nil, err
	}
	return &LabelDirective{base, labels}, nil
}
Variables:
Replaced from ARGs and ENVs from within our stage.
Formats:
LABEL <key>=<value> <key>=<value> <key>=<value> ...
package dockerfile
type LabelDirective struct {
	*baseDirective
	Labels map[string]string
}