build: add experimental support for print flag

Print flag can be used to make additional information
requests about the build and print their results.

Currently Dockerfile supports: outline, targets, subrequests.describe

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/klauspost/compress/huff0/compress.go

@@ -404,6 +404,7 @@ func (s *Scratch) canUseTable(c cTable) bool {
 	return true
 }
 
+//lint:ignore U1000 used for debugging
 func (s *Scratch) validateTable(c cTable) bool {
 	if len(c) < int(s.symbolLen) {
 		return false