[SOLVED] Docker: ubuntu image doesn't keep the packages I installed

Hello everyone,

I am new to docker and it's concepts.

Anyway, I pulled the ubuntu image from docker's online hub via the docker run command.

After it downloaded and it open the bash prompt, I did

apt-get update
apt-get upgrade -y

And I then install some packages. Everything went fine and I then exited the container.

When I return to the ubuntu container to use those programs I've installed, they were not there.

How can I make the ubuntu image keep the packages I install in them?

Thanks