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

Question

I'm not sure if I've misunderstood something here, but it seems like it's only possible to set port mappings by creating a new container from an image. Is there a way to assign a port mapping to an existing Docker container?

Answer

You can change the port mapping by directly editing the hostconfig.json file at /var/lib/docker/containers/[hash_of_the_container]/hostconfig.json or /var/snap/docker/common/var-lib- docker/containers/[hash_of_the_container]/hostconfig.json if you installed Docker as a snap.

You can determine the [hash_of_the_container] via the docker inspect <container_name> command and the value of the "Id" field is the hash.

1. Stop the container (docker stop <container_name>).
2. Stop docker service (per Tacsiazuma's comment)
3. Change the file.
4. Restart your docker engine (to flush/clear config caches).
5. Start the container (docker start <container_name>).

So you don't need to create an image with this approach. You can also change the restart flag here.

P.S. You may visithttps://docs.docker.com/engine/admin/ to learn how to correctly restart your docker engine as per your host machine. I used sudo systemctl restart docker to restart my docker engine that is running on Ubuntu 16.04.