BioMonth - CSC supercomputing and data management for bioscientists

Learning Objectives

In this session, you will be able to:

• See Docker image in action
• Learn where to find pre-made docker images
• Understand the mechanism behind running a docker container

Tutorial Environment using play-with-docker

Within play-with-docker webpage click on create session. Then, another page will open with a big counter on the upper left corner. Click on + add new instance and a terminal like instance should be generated on the right.

Tutorial with Hello-World Container

Let’s start with a simple hello-world container from Dockerhub which is a docker registry for sharing images. Try running the following command on your linux terminal:

docker run hello-world

When a docker run command is issued via commandline interface (CLI), docker client contacts Docker daemon to check whether an image named, “hello-world” exists locally. If the image is not found locally, docker daemon pulls “hello-world” official image from DockerHub (Note: default registry for docker is DockerHub and for other docker registries one has to explicitly mention registry name as well). Once docker image is available locally, docker run command creates a new container from that image. Docker daemon then streams the default output of container to Docker client (= something we see as an output on terminal). Yes…lot of stuff has happened behind the scenes !!!

What is the terminal output from the above docker run command?

  Unable to find image 'hello-world:latest' locally
  latest: Pulling from library/hello-world
  78445dd45222: Pull complete
  Digest: sha256:c5515758d4c5e1e838e9cd307f6c6a0d620b5e07e6f927b07d05f6d12a1ac8d7
  Status: Downloaded newer image for hello-world:latest

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

Note that docker run creates a container, executes the command in it and stops the container when it is done.

What will happen if you run the above command again? Did you observe any change in terminal output now?

Docker image has already been downloaded locally and therefore docker can execute the container straight away.

What is the default image tag used in hello-world example?

By default, an image is pulled with latest tag. It is possible to pull an image with a specified tag and is actually a good practice to use specific tag name for reproducible research. Tag provides a version control-like mechanism for docker images.

Congratulations, you have run a “Hello-World” Docker successfully !!!

Exercises

1. Run a container from an image named “alpine” from DockerHub and execute a command inside that container so that output from the container is “Welcome to CSC !”

2. How would you run the same hello-world example from docker registry with fully qualified reference path for an image? Name few third-party docker registries besides DockerHub.

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