How to port forward in Docker container?

First, running an iptables command during the docker build process would never make sense; even if it worked, the iptables command only modifies the runtime configuration of your kernel. These changes would not persist on the Docker image and would not be available when starting a container.

Second, even if you are running the iptables container after starting a container (rather than when building a container), it will still fail because Docker containers by default do not have the necessary privileges to modify the iptables configuration (or modify networking in general, or mount filesystems, etc). You can start a container with the --privileged flag, but that is probably not what you want to do (because that confers a number of additional privileges on the container which are probably not necessary, and from a security perspective it's a good idea to only grant privileges that are absolutely necessary).

You would typically handle this using Docker's -p option to connect ports on your host to ports in your container, for example:

docker run -p 80:8080 temp

This would link port 80 on your host to port 8080 on the container.

If that's not what you want, an easier solution is just to configure the application in your container to run on the desired port.