When starting a container using run it is possible to expose a port from the container to the host computer. In this case the flag p <host-port>:<container-post> is used:

$ docker run -d -p <host-port>:<container-port> <image-name>
$ docker run --name mongo-container -d mongo -p 27017:27017

It is possible to provide multiple arguments of -p to expose multiple ports.

The expose configuration can also be defined inside a Dockerfile or a docker-compose file.