If you try to access your slim routing with some query string parameter, like
http://foo/bar?parm=balabala, you will probably get a 404 response.
The main reason is, the documented config of slim is telling you to rewrite the uri by:
try_files $uri /app/github/index.php;
try to update it to:
try_files $uri /app/github/index.php?$args;