Fixes #9705 Prevent syntax error in startup.sh

If SESSION_DRIVER is not defined it leads to a syntax error.

docker/startup.sh

@@ -54,7 +54,7 @@ then
   cp -ax /var/www/html/vendor/laravel/passport/database/migrations/* /var/www/html/database/migrations/
 fi
 
-if [ $SESSION_DRIVER = "database" ]
+if [ "$SESSION_DRIVER" = "database" ]
 then
   cp -ax /var/www/html/vendor/laravel/framework/src/Illuminate/Session/Console/stubs/database.stub /var/www/html/database/migrations/2021_05_06_0000_create_sessions_table.php
 fi