Well, your entire app is provisioned at that single server. Your ruby web component, your database, all the platform components (load-balancer, health-monitor, etc.). Right new, each component is inside of a docker container. Docker handles the resource allocation from the host machine.
You can split individual components out into scalable clusters or their own server. But in this case, your web container isn't being given the memory it needs to start the rails console.
to give it that memory, you'd have to scale to a server with more memory.
When you look in you app's dashboard, does it show your web.main is using a high amount of RAM?