Creating containers
Overview
Teaching: 25 min
Exercises: 15 minQuestions
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.
Explain how to create a docker container and get interactive access.
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 host (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
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 I have not explicitly tested this!).
If you want to remove an 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. (Remember that a container is an instance of a particular 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 with the docker container ls command:
$ docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
This command did not return any containers because our containers all exited and thus stopped running after they completed their work.
What containers exist but are not running?
There is also a way to list all known containers, including those that are not currently running, which is to add the --all/-a 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
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 NAME for the container you wish to remove. It will repeat the NAME back to you, if successful.
$ docker container rm zen_dubinsky
zen_dubinsky
Names and IDs
You can use either the
NAMEorCONTAINER IDwith thedocker container rmcommand, whichever you find easier. The names and IDs are randomly generated by Docker to be unique among your list of exisiting containers.
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 successful it will print the full CONTAINER IDs 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 image ls should no longer include the hello-world image.
## Removing all images Removing all images is a little more tricky than removing all containers, you need to combine to commands. The
docker image ls -qcommand returns a list of all image IDs. You can use this to delete all images in one command:docker image rm $(docker image ls -q)
Running containers in the background and interactive access
Often, you will want to use containers to run specific commands, analyses or tasks (as we have seen with the “hello-world” example above). Sometimes, however, you would prefer to have a container running and to get interactive access so you can run commands in the container yourself. You could, for example, be undertaking some data analysis using tools in the container but do not yet know the exact steps you want the analysis to take yet - you are exploring the data.
For this to work, the container image has to have the software installed to support interactive access. Almost all of the images tht are based on Linux distributions will come with this software installed so if you pick, for example, an Ubuntu image then this will work.
First, we need to run the container as we did for hello-world with a couple of additional flags to do two things:
-tflag: Allocate a psuedo-tty to allow us to access the container interactively-dflag: Detach the container and run in the background
$ docker run -t -d ubuntu
Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu
5c939e3a4d10: Pull complete
c63719cdbe7a: Pull complete
19a861ea6baf: Pull complete
651c9d2d6c4f: Pull complete
Digest: sha256:8d31dad0c58f552e890d68bbfb735588b6b820a46e459672d96e585871acc110
Status: Downloaded newer image for ubuntu:latest
dc5fae49118e449662ebdf6ab7790c98b828d2b8def3fca09491bc48c9063580
Some images may also need to define an entry point
For the above appraoch to work the image has to run a shell by default (most Linux OS images do this) for this to work, some minimal OS images may also need to also add the
--entrypoint "/bin/sh"option todocker run.
We can get the image ID and see the image is running in the background with docker ps:
$ docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
dc5fae49118e ubuntu "/bin/bash" 33 seconds ago Up 31 seconds exciting_keller
Now we can start an interactive shell directly in the container image using the docker exec command with the container image ID:
$ docker exec -i -t exciting_keller bash
root@dc5fae49118e:/#
Note that the prompt changed to show we are now in the container image. You can now run commands interactively in the container image.
To get out of the container image, we use the exit command:
root@dc5fae49118e:/# exit
exit
$
Which takes us back to our original command prompt.
Finally, we need to stop the container (it is still running in the background). We use the docker stop command with the container iamge ID to do this:
$ docker container ls
$ docker stop exciting_keller
$ docker container ls
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
dc5fae49118e ubuntu "/bin/bash" 8 minutes ago Up 8 minutes exciting_keller
exciting_keller
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
## YOu will usually want to mount a local directory in the container We will discuss making local directories available in containers in more detail later in the lesson but you will often want to do this with interactive access to containers. As a quick example, to modify the command above so that the container will have the current directory mounts at /data in the container, we would modify the
docker runcommand to be:docker run -t -d -v ${PWD}:/data ubuntu
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 detour to the Docker Hub.
Key Points
Containers are usually created using command line invocations.
The
docker runcommand creates containers from images.The
docker imagecommand can list and manage images that are (now) on your computer.The
docker containercommand can list and manage containers that have been created.The
-d -toptions todocker runallow you to create a container running in the background.The
docker execcommand allows you to execute commands in a running container, including getting interactive access.