Recognize Dockerfile files with extension

Make filenames like `Dockerfile.example` or `Dockerfile.build` also 
open with `dockerfile-mode` automatically.

dockerfile-mode.el

@@ -169,7 +169,7 @@ If prefix arg NO-CACHE is set, don't cache the image."
   (setq local-abbrev-table dockerfile-mode-abbrev-table))
 
 ;;;###autoload
-(add-to-list 'auto-mode-alist '("Dockerfile\\'" . dockerfile-mode))
+(add-to-list 'auto-mode-alist '("Dockerfile\\(?:\\..*\\)?\\'" . dockerfile-mode))
 
 (provide 'dockerfile-mode)