I lose my data when the container exits

When you use docker run to start a container, it actually creates a new container based on the image you have specified.

Besides the other useful answers here, note that you can restart an existing container after it exited and your changes are still there.

docker start f357e2faab77 # restart it in the background
docker attach f357e2faab77 # reattach the terminal & stdin

In addition to Unferth's answer, it is recommended to create a Dockerfile.

In an empty directory, create a file called "Dockerfile" with the following contents.

FROM ubuntu
RUN apt-get install ping
ENTRYPOINT ["ping"]

Create an image using the Dockerfile. Let's use a tag so we don't need to remember the hexadecimal image number.

$ docker build -t iman/ping .

And then run the image in a container.

$ docker run iman/ping stackoverflow.com

You need to commit the changes you make to the container and then run it. Try this:

sudo docker pull ubuntu

sudo docker run ubuntu apt-get install -y ping

Then get the container id using this command:

sudo docker ps -l

Commit changes to the container:

sudo docker commit <container_id> iman/ping

Then run the container:

sudo docker run iman/ping ping www.google.com

This should work.

There are following ways to persist container data:

Docker commit

a) create container from ubuntu image and run a bash terminal.

   $ docker run -i -t ubuntu:14.04 /bin/bash

b) Inside the terminal install curl

   # apt-get update
   # apt-get install curl

c) Exit the container terminal

   # exit

d) Take a note of your container id by executing following command :

   $ docker ps -a

e) save container as new image

   $ docker commit <container_id> new_image_name:tag_name(optional)

f) verify that you can see your new image with curl installed.

   $ docker images           

   $ docker run -it new_image_name:tag_name bash
      # which curl
        /usr/bin/curl

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