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!)

Removing images

If you need to reclaim disk space, you can remove image files. The images and their corresponding containers can start to take up a lot of disk space if you don’t clean them up occasionally. On macOS and Windows, when you uninstall the overall Docker software, it should have the effect of removing all of your image files, although we have not explicitly tested this.

If you want to explicitly remove a container image, you will need to find out details such as the “image ID” or name of the repository. 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
hello-world            latest              fce289e99eb9        4 months ago        1.84kB

We can try to remove remove the hello-world image with a docker image rm command that includes the repository name, like so but…:

$ docker image rm hello-world

Error response from daemon: conflict: unable to remove repository reference "hello-world" (must force) - container e7d3b76b00f4 is using its referenced image fce289e99eb9

We get this error because there are containers created that depend on this image.

What containers are running?

As indicated by the error above there is still an existing container from this image. We can list the running containers by typing.

$ docker container ls

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

The first version of the command is orthogonal to what we know from the Unix shell but we might want to use the shorter version. For reference, “ps” stands for “Process Status”. We should also notice that this command didn’t return any containers because our containers all exited and thus stopped running after they completed their work.

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/-a flag to the docker container ls/docker ps 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

We will talk more about how you might use these exited containers and how to restart a container later in this lesson.

How do I remove an exited container?

To delete an exited container you can run the following command, inserting the CONTAINER ID for the container you wish to remove. It will repeat the CONTAINER ID back to you, if successful.

$ docker container rm 9c698655416a

9c698655416a

If you want to remove all exited containers at once you can use the docker containers prune command. Be careful with this command. If you have containers you may want to reconnect to, you should not use this command. It will ask you if to confirm you want to remove these containers, see output below. If successfull it will print the full CONTAINER ID back to you.

$ docker container prune

WARNING! This will remove all stopped containers.
Are you sure you want to continue? [y/N] y
Deleted Containers:
9c698655416a848278d16bb1352b97e72b7ea85884bff8f106877afe0210acfc
6dd822cf6ca92f3040eaecbd26ad2af63595f30bb7e7a20eacf4554f6ccc9b2b

Removing images, for real this time

$ docker image rm hello-world

Untagged: hello-world:latest
Untagged: hello-world@sha256:5f179596a7335398b805f036f7e8561b6f0e32cd30a32f5e19d17a3cda6cc33d
Deleted: sha256:fce289e99eb9bca977dae136fbe2a82b6b7d4c372474c9235adc1741675f587e
Deleted: sha256:af0b15c8625bb1938f1d7b17081031f649fd14e6b233688eea3c5483994a66a3

The reason that there are a few lines of output, is that a given image may have been formed by merging multiple underlying layers. Any layers that are used by multiple Docker images will only be stored once. Now the result of docker images should no longer include the hello-world image.

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.

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.

Key Points

• Containers are usually created using command line invocations.

• The docker run command creates containers from images.

• The docker image command can list and manage images that are (now) on your computer.

• The docker container command can list and manage containers that have been created.