Some fixed for new LABEL stuff

- command.Commands was missing "Label"
- used the correct error string in dispatcher when LABEL has no args, otherwise
  the test TestBuildMissingArgs will not work
- removed the premature error msg in line_parser that was blocking the
  label() func in dispatcher from showing the err msg in previous bullet
- since LABEL uses the env parser it needs to be added to the replaceEnvAllowed
  list so that proper quote processing will be done.  Especially once
  PR #10431 is merged.

Signed-off-by: Doug Davis <dug@us.ibm.com>

builder/command/command.go

@@ -22,6 +22,7 @@ const (
 // Commands is list of all Dockerfile commands
 var Commands = map[string]struct{}{
 	Env:        {},
+	Label:      {},
 	Maintainer: {},
 	Add:        {},
 	Copy:       {},