Fix .dockerignore for build context copies in later stages (#1447)

* Extend .dockerignore integration test with copies in later stages

.dockerignore should continue to apply when copying from the build context in later stages, but it currently doesn't

* Replace excluded global with passed along FileContext struct

This new FileContext struct allows much cleaner handling of context specific file exclusions.
The global excluded file state is no longer needed.

Additionally this also fixes the issue where excluded files aren't being applied for build context copies in later build stages.

pkg/executor/composite_cache.go

@@ -55,7 +55,7 @@ func (s *CompositeCache) Hash() (string, error) {
 	return util.SHA256(strings.NewReader(s.Key()))
 }
 
-func (s *CompositeCache) AddPath(p, context string) error {
+func (s *CompositeCache) AddPath(p string, context util.FileContext) error {
 	sha := sha256.New()
 	fi, err := os.Lstat(p)
 	if err != nil {
@@ -70,13 +70,13 @@ func (s *CompositeCache) AddPath(p, context string) error {
 
 		// Only add the hash of this directory to the key
 		// if there is any ignored content.
-		if !empty || !util.ExcludeFile(p, context) {
+		if !empty || !context.ExcludesFile(p) {
 			s.keys = append(s.keys, k)
 		}
 		return nil
 	}
 
-	if util.ExcludeFile(p, context) {
+	if context.ExcludesFile(p) {
 		return nil
 	}
 	fh, err := util.CacheHasher()(p)
@@ -92,14 +92,14 @@ func (s *CompositeCache) AddPath(p, context string) error {
 }
 
 // HashDir returns a hash of the directory.
-func hashDir(p, context string) (bool, string, error) {
+func hashDir(p string, context util.FileContext) (bool, string, error) {
 	sha := sha256.New()
 	empty := true
 	if err := filepath.Walk(p, func(path string, fi os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
-		exclude := util.ExcludeFile(path, context)
+		exclude := context.ExcludesFile(path)
 		if exclude {
 			return nil
 		}