make /start executable

While trying to use https://github.com/amtrack/buildpack-gitlab to build a gitlab container, I got an error that I tracked down to /start not being executable. I couldn't see any other way that  the start script written here would be marked +x before the container tried to execute it, so I added this.

(If I missed something, don't hesitate to reject this -- I'm new to dokku. Thanks!)