Fix ReadAll to run on Windows.

filepath.Clean converts filenames to filenames with native path
separators. Use ToSlash to normalize.

Signed-off-by: Anusha Ragunathan <anusha@docker.com>

builder/dockerignore/dockerignore.go

@@ -25,6 +25,7 @@ func ReadAll(reader io.ReadCloser) ([]string, error) {
 			continue
 		}
 		pattern = filepath.Clean(pattern)
+		pattern = filepath.ToSlash(pattern)
 		excludes = append(excludes, pattern)
 	}
 	if err := scanner.Err(); err != nil {