This is fine for a development environment or even a very small application. But when you need to scale individual components of the stack differently, e.g Post

This is fine for a development environment or even a very small application. But when you need to scale individual components of the stack differently, e.g Postgres, Redis and RabbitMQ, it doesn't make sense to scale the whole stack horizontally by simply starting a new container. If you take a look at PaaS such as Heroku you are always getting a DNS address to these other services. In some cases they may be simply another scheduled container, but in other cases you may be using a database as a service platform similar to Amazon Dynamo.