Deploy from sub-directory of Git repository

I'm brand new to Forge and trying to setup my project to deploy from a subdirectory of our Git repo. Our application is stored in a "bin" folder within the root of the repo.

Here is the default Forge deploy script...

cd /home/forge/default

git pull origin master

composer install

php artisan migrate --force

How could I modify this to pull specifically from the "bin" folder?