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/moby/buildkit/client/llb/sourcemap.go

@@ -61,7 +61,7 @@ func (smc *sourceMapCollector) Add(dgst digest.Digest, ls []*SourceLocation) {
 		}
 		smc.index[l.SourceMap] = idx
 	}
-	smc.locations[dgst] = ls
+	smc.locations[dgst] = append(smc.locations[dgst], ls...)
 }
 
 func (smc *sourceMapCollector) Marshal(ctx context.Context, co ...ConstraintsOpt) (*pb.Source, error) {

vendor/github.com/moby/buildkit/client/llb/state.go

@@ -199,10 +199,10 @@ func marshal(ctx context.Context, v Vertex, def *Definition, s *sourceMapCollect
 	if opMeta != nil {
 		def.Metadata[dgst] = mergeMetadata(def.Metadata[dgst], *opMeta)
 	}
+	s.Add(dgst, sls)
 	if _, ok := cache[dgst]; ok {
 		return def, nil
 	}
-	s.Add(dgst, sls)
 	def.Def = append(def.Def, dt)
 	cache[dgst] = struct{}{}
 	return def, nil