Demonstration on Docker containers

Demonstration on Docker containers

Resources

Please install Docker on your laptop! See this for Windows and MacOS. For Linux users you should easily find a documentation online according to your distribution!

Check your installation

Open a terminal
try docker --help

Pull an Ubuntu basic image from Docker Hub

take a look at the image on Docker Hub
docker pull ubuntu You’ve just pull an image from Docker Hub!
docker image ls You can see the Ubuntu image on your list of images.

Create a Dockerfile

A Dockerfile is a file containing a set of instruction to build a new Docker image. Common Dockerfile instructions start with RUN, ENV, FROM, MAINTAINER, ADD, and CMD, among others. They are equivalent to system commands. See the documentation for more details.

create a new Dockerfile touch your-path/Dockerfile
open it with your favorite text editor (emacs, vi, sublimetext etc.)
copy/paste the following content in the Dockerfile

FROM ubuntu:latest

RUN apt-get -y update
RUN apt-get install sl -y

The instruction FROM ubuntu:latest means that the new Docker image will be built by using ubuntu:latest as a basic layer;
the other instructions run commands on the basic image before the new layers and image are created, in this example the packages sl (remember the train?) is installed.

Build a new Docker image and run it

docker build -f Dockerfile -t demo-ubuntu . This command builds the new image, called demo-ubuntu, described in the Dockerfile Dockerfile.
docker run -i -t demo-ubuntu /usr/games/sl Have you seen the train? Yes ! It means your new image have been successfully created and that you have been able to run a command on a container that runs this image.

Get a bash on your container

docker run -i -t demo-ubuntu /bin/bash You are know using a bash terminal on your container.
apt-get install python Python is not installed in the basic Ubuntu image. Let’s install it directly from the bash terminal on your container.
python --help Ok Python have been installed successfully on the container!
exit

WTF?

docker run -i -t demo-ubuntu python --help What? You’ve just installed Python in your container but it is not working anymore? What is happening?

Do you remember the philosophy of Docker about application packaging? Do you remember the difference with LXC? You’ve actually created another container from the image demo-ubuntu with this command, you have not run the command on the one created before.

docker ps -a Now you can see the container you’ve created so far. Docker has given a name to each container you’ve created (last column).
docker restart [name] You can restart the previous container.
docker attach [name] And you can attach to this container to get a bash on it.
python --help It should work! But still this illustrates the ephemeral nature of Docker containers.
exit
docker exec [name] ls This executes a command ls on the container [name].
docker rm [name] You’ve just removed the container [name].

Data volume

Let’s create another image with an associated volume. Data volumes is the cleanest way to handle persistent data within Docker containers.

Create another Dockerfile toto with the following content:

FROM ubuntu:latest

RUN apt-get -y update
RUN apt-get install sl -y
RUN apt-get -y install python

docker build -f toto -t demo-ubuntu2 .
docker run -i -t demo-ubuntu2 python --help Python is installed in this container as the new Dockerfile included it
docker run -it -v /data --name container2 demo-ubuntu2 You could also use separately docker volume create [name] to create a new volume.
ls You should see the directory data mounted in your container.
touch data/test.org You’ve created a file in data.
exit

docker inspect container2 You should see the volume details:

"Mounts": [
         {
             "Type": "volume",
             "Name": "a4a840fe25c23c855fcf7f546c4a45b09b1580a39d0203df4630d1d1dd9281b3",
             "Source": "/var/lib/docker/volumes/a4a840fe25c23c855fcf7f546c4a45b09b1580a39d0203df4630d1d1dd9281b3/_data",
             "Destination": "/data",
             "Driver": "local",
             "Mode": "",
             "RW": true,
             "Propagation": ""
         }
     ],

docker retsart container2
docker attach container2
ls data/
exit
docker rm container2 You’ve deleted the container2.
docker volume ls The volume is still visible in the list of volumes. If you add -v to the removal of your container, the associated volume would be removed as well.
docker run -it -v [volumne-name]:/app --name container3 demo-ubuntu The volume [volume-name] is mounted on the new container3 and placed in the directory app.
ls app/ You should find you previous file test.org.