Short answer is: you need to add (?d) making it (?d).DS_Store
Thanks @maelcum for your post Useful .stignore Patterns - #69 by maelcum
and the relevant information from the RTFM:
A pattern beginning with a (?d) prefix enables removal of these files if they are preventing directory deletion. This prefix should be used by any OS generated files which you are happy to be removed.