Allow referencing previous stages as source in FROM (#38)

Signed-off-by: Kathryn Baldauf <[email protected]>

Author: katiewasnothere

pkg/build/frontend.go

@@ -129,7 +129,7 @@ func resolveStates(ctx context.Context, bopts *BOpts, platform ocispecs.Platform
 	states := map[string]stateMeta{}
 	stateLock := sync.Mutex{}
 
-	for _, stage := range stages {
+	for i, stage := range stages {
 		wg.Add(1)
 		go func(stage instructions.Stage) {
 			defer wg.Done()
@@ -145,6 +145,12 @@ func resolveStates(ctx context.Context, bopts *BOpts, platform ocispecs.Platform
 				return
 			}
 
+			// if there's another stage with this name before the current stage, that will be used as the source
+			namedIndex, hasNamedStage := instructions.HasStage(stages, resolvedBaseStageName.Result)
+			if hasNamedStage && namedIndex < i {
+				return
+			}
+
 			ref, err := dref.ParseAnyReference(resolvedBaseStageName.Result)
 			if err != nil {
 				if err == reference.ErrObjectRequired {