Fix caching to respect .dockerignore

Previously kaniko would compute the cache key for any copy command by computing
the combined hash of all files in a directory, even if they were listed
as ignored.

With this change, the cache key creation was updated to be aware of ignored
files.

Related issues:
* https://github.com/GoogleContainerTools/kaniko/issues/594

pkg/executor/build.go

@@ -158,8 +158,10 @@ func (s *stageBuilder) populateCompositeKey(command fmt.Stringer, files []string
 		compositeKey = s.populateCopyCmdCompositeKey(command, v.From(), compositeKey)
 	}
 
+	srcCtx := s.opts.SrcContext
+
 	for _, f := range files {
-		if err := compositeKey.AddPath(f); err != nil {
+		if err := compositeKey.AddPath(f, srcCtx); err != nil {
 			return compositeKey, err
 		}
 	}

pkg/executor/composite_cache.go

@@ -54,14 +54,18 @@ func (s *CompositeCache) Hash() (string, error) {
 	return util.SHA256(strings.NewReader(s.Key()))
 }
 
-func (s *CompositeCache) AddPath(p string) error {
+func (s *CompositeCache) AddPath(p, context string) error {
 	sha := sha256.New()
 	fi, err := os.Lstat(p)
 	if err != nil {
 		return err
 	}
+	if util.ExcludeFile(p, context) {
+		return os.ErrNotExist
+	}
+
 	if fi.Mode().IsDir() {
-		k, err := HashDir(p)
+		k, err := HashDir(p, context)
 		if err != nil {
 			return err
 		}
@@ -81,12 +85,20 @@ func (s *CompositeCache) AddPath(p string) error {
 }
 
 // HashDir returns a hash of the directory.
-func HashDir(p string) (string, error) {
+func HashDir(p, context string) (string, error) {
 	sha := sha256.New()
 	if err := filepath.Walk(p, func(path string, fi os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
+		exclude := util.ExcludeFile(path, context)
+		if fi.IsDir() && exclude {
+			return filepath.SkipDir
+		}
+		if exclude {
+			return nil
+		}
+
 		fileHash, err := util.CacheHasher()(path)
 		if err != nil {
 			return err