Creating containers

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How do I get Docker to perform computation?

Objectives

• Demonstrate how to create an instance of a container from an image.

• Explain how to list (container) images on your laptop.

• Explain how to list running and completed containers.

Creating a “hello world” container

Reminder of terminology: images and containers

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

One of the simplest Docker container images just allows you to create containers that print a welcome message.

To create and run containers from named Docker images you use the docker run command. Open a shell window if you do not already have one open and try the following docker run invocation. Note that it does not matter what your current working directory is.

$ docker run hello-world

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
1b930d010525: Pull complete 
Digest: sha256:2557e3c07ed1e38f26e389462d03ed943586f744621577a99efb77324b0fe535
Status: Downloaded newer image for hello-world:latest

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.
    (amd64)
 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.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

Now try to run the above command again, and observe the difference in the output.

The message from “Hello from Docker!” is what’s produced by instances of the hello-world container. It should be identical every time you make an instance of the hello-world container and run it.

The first time you saw some initial output from the Docker command itself:

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
1b930d010525: Pull complete 
Digest: sha256:2557e3c07ed1e38f26e389462d03ed943586f744621577a99efb77324b0fe535
Status: Downloaded newer image for hello-world:latest

What this says is that your laptop didn’t have its own local copy of the hello-world container’s image.

Docker then connected to the Docker Hub to find and download the (container) image with that name.

The message notes information about the Docker Hub website, but we’ll return to that in the next episode.

The “digest” is a secure fingerprint (a “hash”) of the particular version of the container image that you now have… (Although of course the usefulness of that hash is minimal if you don’t know what to compare it to!)

Where did that “hello-world” container image get stored?

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

$ docker image ls

REPOSITORY             TAG                 IMAGE ID            CREATED             SIZE
hello-world            latest              fce289e99eb9        4 weeks ago         1.84kB

(Actually, I had some additional images on my laptop that I’ve removed from the above, and in live demo I’m likely to end up displaying a great many more lines!)

If you need to reclaim space, you will need to remove image files. On macOS and Windows, when you uninstall the overall Docker software, it should have the effect of removing all of your image files, although I have not explicitly tested this.

Removing images

If you want to explicitly remove a container image, you will need to find out details such as the “image ID”. For example say my laptop contained the following image.

$ docker image ls

REPOSITORY             TAG                 IMAGE ID            CREATED             SIZE
ubuntu                 trusty              dc4491992653        12 months ago       222MB

I can remove the image with a docker image rm command that includes the image ID, such as:

$ docker image rm dc4491992653

Untagged: ubuntu:trusty
Untagged: ubuntu@sha256:e1c8bff470c771c6e86d3166607e2c74e6986b05bf339784a9cab70e0e03c7c3
Deleted: sha256:dc4491992653ecf02ae2d0e9d3dbdaab63af8ccdcab87ee0ee7e532f7087dd73
Deleted: sha256:1239c33230909cc231da97b851df65e252dc9811dcee2af0ecf3b225e2805a31
Deleted: sha256:ce4caf69568d9109febd1f5307b62d85ab84e7a947fded041be49c847b412e5a
Deleted: sha256:4c711cc0452303f0fb6ce885c84130e32bb649b03f690fd0e4626a874b1cc8cf
Deleted: sha256:a375921af0e34b1cb09a35af24265db01b1eb65edabadaf70d56505a60a6de2b
Deleted: sha256:c41b9462ea4bbc2f578e42bd915214d54948d960b9b8c6815daf8586811c2d38

The reason that there is so much output, is that a given image may be created by merging multiple underlying layers. Any layers that are used by multiple Docker images will only be stored once.

What containers are running?

The command to list the containers that are currently running is

docker container ls

… although the output should be blank, since the instances of hello-world containers will have been wiped away as soon as they completed. However the container image remains on your computer so that it is quick for you to create future such instances of hello-world containers.

What containers have run recently?

There is also a way to list running containers, and those that have completed recently, which is to add the --all flag to the docker container ls command as shown below.

$ docker container ls --all

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                     PORTS               NAMES
9c698655416a        hello-world         "/hello"            2 minutes ago       Exited (0) 2 minutes ago                       zen_dubinsky
6dd822cf6ca9        hello-world         "/hello"            3 minutes ago       Exited (0) 3 minutes ago                       eager_engelbart

Removing containers

Just as there is a docker image rm command for removing images, there is a docker container rm command for removing containers. We should need to use that command in this session, though.

Conclusion

You have now successfully acquired a Docker image file to your computer, and have created a Docker container from it. While this already effects a reproducible computational environment, the image contents are not under your control, so we look at this topic, after a quick discussion about the Docker Hub.

The Docker official documentation is helpful!

There is lots of great documentation at https://docs.docker.com/, for example, detailed reference material and tutorials covering the use of the commands mentioned above.

Key Points

• Containers are usually created using command line invocations.

• The docker run command creates containers from images.

• The docker image command lists images that are (now) on your computer.

• The docker container command lists containers that have been created.