Docker: Internal Ports

Question

I'm new to docker and wondering why so many examples of docker-compose map ports from internal to external instead of just keeping them the same. For example, one postgres example I saw mapped 4612:5432.

Is there a reason people don't just keep it 5432:5432?

Answer 1

There may be different reasons not to map to same port on host: doing this allows to have multiple Postgres running on separate ports and tweak them in case for some reason you want different ports in DEV/PROD.

Answer 2

For local dev env, except for port conflicting issues, there is no other reason. Unlike exposing ports to internet, there is no security reasons for container port mapping and exposing.