This fix tries to address issues in #23221 where Dockerfile
may consists of UTF-8 BOM. This likely happens when Notepad
tries to save a file as UTF-8 in Windows.
This fix skips the UTF-8 BOM bytes from the beginning of the
Dockerfile if exists.
Additional tests has been added to cover the changes in this
fix.
This fix fixes#23221.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>