How to install "ifconfig" command in my ubuntu docker image?

On a fresh ubuntu docker image, run

apt-get update
apt-get install net-tools

These can be executed by logging into the docker container or add this to your dockerfile to build an image with the same.

You could also consider:

RUN apt-get update && apt-get install -y iputils-ping

(as Contango comments: you must first run apt-get update, to avoid error with missing repository).

See "Replacing ifconfig with ip"

it is most often recommended to move forward with the command that has replaced ifconfig. That command is ip, and it does a great job of stepping in for the out-of-date ifconfig.

But as seen in "Getting a Docker container's IP address from the host", using docker inspect can be more useful depending on your use case.