zlacker

-p exposes the port from the container on a specific port on the host machine. -P does the same, but for all ports listed as exposed in the container.

If you just run a container, it will expose zero ports, regardless of any config made in the Docker image or container.

The way you're supposed to use Docker is to create a Docker network, attach the various containers there, and expose only the ports on specific containers that you need external access to. All containers in any network can connect to each other, with zero exposed external ports.

The trouble is just that this is not really explained well for new users, and so ends up being that aforementioned foot gun.