How do i expose a port in a docker container?

shoebill

$ docker run -d -p 80:80  nginx                                              
8d519e1896c0cd7f01f9c799bc5a874daea301abc77822ab1cd6b6b2eab2dff0

The -d is detach mode, runs in the background.
The -p publishes the port(s) to the container.

If you go to the host browser: http://localhost, you will see the web server welcome banner.

Also curl to the URL will work.

$ curl http://localhost

If no mapped port is specified a random port is mapped on the host server, where the docker command is executed.

$ docker run -d -p 80 nginx                                                   
73484a41ea8e42029d3a8801479ea73bda290c435ee5048fad5d3f7ad1ead028

Docker inspect will show the host port.

$ docker inspect 73484a41ea8e

...
...
...
"Ports": {
                "80/tcp": [
                    {
                        "HostIp": "0.0.0.0",
                        "HostPort": "32771"
                    }
                ]
            },
...
...
...

Test the port mapping.

$ curl http://localhost