How do I assign a port mapping to an existing Docker container?

I’m also interested in this problem.

As @Thasmo mentioned, port forwardings can be specified ONLY with docker run (and docker create) command.
Other commands, docker start does not have -p option and docker port only displays current forwardings.

To add port forwardings, I always follow these steps,

1. stop running container

   docker stop test01
   

2. commit the container

   docker commit test01 test02
   

   NOTE: The above, test02 is a new image that I’m constructing from the test01 container.

3. re-run from the commited image

   docker run -p 8080:8080 -td test02
   

Where the first 8080 is the local port and the second 8080 is the container port.