PDOException SQLSTATE HY000 2002 No such file or directory

Question

My (very basic) site appears to have been successfully deployed to FortRabbit, but whenever I use SSH to run some tasks (like php artisan migrate or php artisan db:seed), I receive an error message:

[PDOException]
SQLSTATE[HY000] [2002] No such file or directory

At some point the migration must have worked, because my tables are there - but this doesn't explain why it isn't working for me now.