Optimize .dockerignore when there are exclusions

Closes #20470

Before this PR we used to scan the entire build context when there were
exclusions in the .dockerignore file (paths that started with !). Now we
only traverse into subdirs when one of the exclusions starts with that dir
path.

Signed-off-by: Doug Davis <dug@us.ibm.com>

archive/archive.go

@@ -582,10 +582,36 @@ func TarWithOptions(srcPath string, options *TarOptions) (io.ReadCloser, error)
 				}
 
 				if skip {
-					if !exceptions && f.IsDir() {
+					// If we want to skip this file and its a directory
+					// then we should first check to see if there's an
+					// excludes pattern (eg !dir/file) that starts with this
+					// dir. If so then we can't skip this dir.
+
+					// Its not a dir then so we can just return/skip.
+					if !f.IsDir() {
+						return nil
+					}
+
+					// No exceptions (!...) in patterns so just skip dir
+					if !exceptions {
 						return filepath.SkipDir
 					}
-					return nil
+
+					dirSlash := relFilePath + string(filepath.Separator)
+
+					for _, pat := range patterns {
+						if pat[0] != '!' {
+							continue
+						}
+						pat = pat[1:] + string(filepath.Separator)
+						if strings.HasPrefix(pat, dirSlash) {
+							// found a match - so can't skip this dir
+							return nil
+						}
+					}
+
+					// No matching exclusion dir so just skip dir
+					return filepath.SkipDir
 				}
 
 				if seen[relFilePath] {