Introduction to Docker

There are many container runtime available out there. One of popular runtimes is Docker. Docker has proven to an extraordinary tool for developers and researchers alike. Most of today’s workshop will be spent of Docker command line (CLI) utility.

Downloading Docker images

The docker image command is used to list and modify Docker images. You can find out what container images you have on your computer by using the following command (“ls” is short for “list”):

docker image ls

If you’ve just installed Docker, you won’t see any images listed.

To get a copy of the hello-world Docker image from the internet, run this command:

docker pull hello-world

You should see output like this:

Using default tag: latest
latest: Pulling from library/hello-world
93288797bd35: Pull complete
Digest: sha256:975f4b14f326b05db86e16de00144f9c12257553bba9484fed41f9b6f2257800
Status: Downloaded newer image for hello-world:latest
docker.io/library/hello-world:latest

DockerHub

Where did the hello-world image come from? It came from the DockerHub website, which is a place to share Docker images with other people. More on that in a later episode.

Exercise: Check on Your Images

What command would you use to see if the hello-world Docker image had downloaded successfully and was on your computer? Give it a try before checking the solution.

Solution

To see if the hello-world image is now on your computer, run:

docker image ls

Note that the downloaded hello-world image is not in the folder where you are in the terminal! (Run ls by itself to check.) The image is not a file like our normal programs and files; Docker stores it in a specific location that isn’t commonly accessed, so it’s necessary to use the special docker image command to see what Docker images you have on your computer.

Running the hello-world container

To create and run containers from named Docker images you use the docker run command. Try the following docker run invocation. Note that it does not matter what your current working directory is.

docker run hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:

1. The Docker client contacted the Docker daemon.

2. The Docker daemon pulled the “hello-world” image from the Docker Hub. (arm64v8)

3. The Docker daemon created a new container from that image which runs the executable that produces the output you are currently reading.

4. The Docker daemon streamed that output to the Docker client, which sent it to your terminal.

The hello-world container is set up to run an action by default - namely to print this message.

Using docker run to get the image

We could have skipped the docker pull step; if you use the docker run command and you don’t already have a copy of the Docker image, Docker will automatically pull the image first and then run it.

Running a container with a chosen command

But what if we wanted to do something different with the container? The output just gave us a suggestion of what to do – let’s use a different Docker image to explore what else we can do with the docker run command. The suggestion above is to use ubuntu, but we’re going to run a different type of Linux, alpine instead because it’s quicker to download.

Run the Alpine Docker container

Try downloading and running the alpine Docker container. You can do it in two steps, or one. What are they?

What happened when you ran the Alpine Docker container?

docker run alpine

If you never used the alpine docker image on your computer, docker probably printed a message that it couldn’t find the image and had to download it. If you used the alpine image before, the command will probably show no output. That’s because this particular container is designed for you to provide commands yourself. Try running this instead:

docker run alpine cat /etc/os-release

You should see the output of the cat /etc/os-release command, which prints out the version of Alpine Linux that this container is using and a few additional bits of information.

Exercise: Hello World, Part 2

Can you run the container and make it print a “hello world” message? Give it a try before checking the solution.

Solution

Use the same command as above, but with the echo command to print a message.

docker run alpine echo ‘Hello World’

So here, we see another option – we can provide commands at the end of the docker run command and they will execute inside the running container.

Running containers interactively

In all the examples above, Docker has started the container, run a command, and then immediately shut down the container. But what if we wanted to keep the container running so we could log into it and test drive more commands? The way to do this is by adding the interactive flag -it to the docker run command and by providing a shell (usually bash or sh) as our command. The alpine docker image doesn’t include bash so we need to use sh.

docker run -it alpine sh

Your prompt should change significantly to look like this:

/ #

That’s because you’re now inside the running container! Try these commands:

• pwd

• ls

• whoami

• echo $PATH

• cat /etc/os-release

All of these are being run from inside the running container, so you’ll get information about the container itself, instead of your computer. To finish using the container, just type exit.

Reminder of terminology: images and containers

Recall that a container “image” is the template from which particular instances of containers will be created.

Let’s explore our first Docker container. The Docker team provides a simple container image online called hello-world. We’ll start with that one.