Introduction
Docker is an application that simplifies the process of managing application processes in containers. Containers let you run your applications in resource-isolated processes. They’re similar to virtual machines, but containers are more portable, more resource-friendly, and more dependent on the host operating system.
For a detailed introduction to the different components of a Docker container, check out The Docker Ecosystem: An Introduction to Common Components.
In this tutorial, you’ll install and use Docker Community Edition (CE) on Ubuntu 20.04. You’ll install Docker itself, work with containers and images, and push an image to a Docker Repository.
Note
This article will walk you through installing Docker on an Ubuntu server. If you wanted a 1-click way to deploy a Docker application to a live server, take a look at DigitalOcean App Platform.
Prerequisites
To follow this tutorial, you will need the following:
One Ubuntu 20.04 server set up by following the Ubuntu 20.04 initial server setup guide, including a sudo non-root user and a firewall.
An account on Docker Hub if you wish to create your own images and push them to Docker Hub, as shown in Steps 7 and 8.
Step 1 — Installing Docker
The Docker installation package available in the official Ubuntu repository may not be the latest version. To ensure we get the latest version, we’ll install Docker from the official Docker repository. To do that, we’ll add a new package source, add the GPG key from Docker to ensure the downloads are valid, and then install the package.
First, update your existing list of packages:
sudo apt update
Next, install a few prerequisite packages which let apt use packages over HTTPS:
sudo apt install apt-transport-https ca-certificates curl software-properties-common
Then add the GPG key for the official Docker repository to your system:
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
Add the Docker repository to APT sources:
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu focal stable"
This will also update our package database with the Docker packages from the newly added repo.
Make sure you are about to install from the Docker repo instead of the default Ubuntu repo:
apt-cache policy docker-ce
You’ll see output like this, although the version number for Docker may be different:
of apt-cache policy docker-ce
docker-ce:
Installed: (none)
Candidate: 5:19.03.9~3-0~ubuntu-focal
Version table:
5:19.03.9~3-0~ubuntu-focal 500
500 https://download.docker.com/linux/ubuntu focal/stable amd64 Packages
Notice that docker-ce is not installed, but the candidate for installation is from the Docker repository for Ubuntu 20.04 (focal).
Finally, install Docker:
sudo apt install docker-ce
Docker should now be installed, the daemon started, and the process enabled to start on boot. Check that it’s running:
sudo systemctl status docker
The output should be similar to the following, showing that the service is active and running:
● docker.service - Docker Application Container Engine
Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
Active: active (running) since Tue 2020-05-19 17:00:41 UTC; 17s ago
TriggeredBy: ● docker.socket
Docs: https://docs.docker.com
Main PID: 24321 (dockerd)
Tasks: 8
Memory: 46.4M
CGroup: /system.slice/docker.service
└─24321 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock
Installing Docker now gives you not just the Docker service (daemon) but also the docker command line utility, or the Docker client. We’ll explore how to use the docker command later in this tutorial.
Step 2 — Executing the Docker Command Without Sudo (Optional)
By default, the docker command can only be run the root user or by a user in the docker group, which is automatically created during Docker’s installation process. If you attempt to run the docker command without prefixing it with sudo or without being in the docker group, you’ll get an output like this:
docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.
If you want to avoid typing sudo whenever you run the docker command, add your username to the docker group:
sudo usermod -aG docker ${USER}
To apply the new group membership, log out of the server and back in, or type the following:
su - ${USER}
You will be prompted to enter your user’s password to continue.
Confirm that your user is now added to the docker group by typing:
groups
sammy sudo docker
If you need to add a user to the docker group that you’re not logged in as, declare that username explicitly using:
sudo usermod -aG docker username
The rest of this article assumes you are running the docker command as a user in the docker group. If you choose not to, please prepend the commands with sudo.
Let’s explore the docker command next.
Step 3 — Using the Docker Command
Using docker consists of passing it a chain of options and commands followed by arguments. The syntax takes this form:
docker [option] [command] [arguments]
To view all available subcommands, type:
docker
As of Docker 19, the complete list of available subcommands includes:
attach Attach local standard input, output, and error streams to a running container
build Build an image from a Dockerfile
commit Create a new image from a container's changes
cp Copy files/folders between a container and the local filesystem
create Create a new container
diff Inspect changes to files or directories on a container's filesystem
events Get real time events from the server
exec Run a command in a running container
export Export a container's filesystem as a tar archive
history Show the history of an image
images List images
import Import the contents from a tarball to create a filesystem image
info Display system-wide information
inspect Return low-level information on Docker objects
kill Kill one or more running containers
load Load an image from a tar archive or STDIN
login Log in to a Docker registry
logout Log out from a Docker registry
logs Fetch the logs of a container
pause Pause all processes within one or more containers
port List port mappings or a specific mapping for the container
ps List containers
pull Pull an image or a repository from a registry
push Push an image or a repository to a registry
rename Rename a container
restart Restart one or more containers
rm Remove one or more containers
rmi Remove one or more images
run Run a command in a new container
save Save one or more images to a tar archive (streamed to STDOUT by default)
search Search the Docker Hub for images
start Start one or more stopped containers
stats Display a live stream of container(s) resource usage statistics
stop Stop one or more running containers
tag Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE
top Display the running processes of a container
unpause Unpause all processes within one or more containers
update Update configuration of one or more containers
version Show the Docker version information
wait Block until one or more containers stop, then print their exit codes
To view the options available to a specific command, type:
docker docker-subcommand --help
To view system-wide information about Docker, use:
docker info
Let’s explore some of these commands. We’ll start by working with images.
Step 4 — Working with Docker Images
Docker containers are built from Docker images. By default, Docker pulls these images from Docker Hub, a Docker registry managed by Docker, the company behind the Docker project. Anyone can host their Docker images on Docker Hub, so most applications and Linux distributions you’ll need will have images hosted there.
To check whether you can access and download images from Docker Hub, type:
docker run hello-world
The output will indicate that Docker in working correctly:
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
0e03bdcc26d7: Pull complete
Digest: sha256:6a65f928fb91fcfbc963f7aa6d57c8eeb426ad9a20c7ee045538ef34847f44f1
Status: Downloaded newer image for hello-world:latest
Hello from Docker!
This message shows that your installation appears to be working correctly.
...
Docker was initially unable to find the hello-world image locally, so it downloaded the image from Docker Hub, which is the default repository. Once the image downloaded, Docker created a container from the image and the application within the container executed, displaying the message.
You can search for images available on Docker Hub by using the docker command with the search subcommand. For example, to search for the Ubuntu image, type:
docker search ubuntu
The script will crawl Docker Hub and return a listing of all images whose name match the search string. In this case, the output will be similar to this:
NAME DESCRIPTION STARS OFFICIAL AUTOMATED
ubuntu Ubuntu is a Debian-based Linux operating sys… 10908 [OK]
dorowu/ubuntu-desktop-lxde-vnc Docker image to provide HTML5 VNC interface … 428 [OK]
rastasheep/ubuntu-sshd Dockerized SSH service, built on top of offi… 244 [OK]
consol/ubuntu-xfce-vnc Ubuntu container with "headless" VNC session… 218 [OK]
ubuntu-upstart Upstart is an event-based replacement for th… 108 [OK]
ansible/ubuntu14.04-ansible Ubuntu 14.04 LTS with
...
In the OFFICIAL column, OK indicates an image built and supported by the company behind the project. Once you’ve identified the image that you would like to use, you can download it to your computer using the pull subcommand.
Execute the following command to download the official ubuntu image to your computer:
docker pull ubuntu
You’ll see the following output:
Using default tag: latest
latest: Pulling from library/ubuntu
d51af753c3d3: Pull complete
fc878cd0a91c: Pull complete
6154df8ff988: Pull complete
fee5db0ff82f: Pull complete
Digest: sha256:747d2dbbaaee995098c9792d99bd333c6783ce56150d1b11e333bbceed5c54d7
Status: Downloaded newer image for ubuntu:latest
docker.io/library/ubuntu:latest
After an image has been downloaded, you can then run a container using the downloaded image with the run subcommand. As you saw with the hello-world example, if an image has not been downloaded when docker is executed with the run subcommand, the Docker client will first download the image, then run a container using it.
To see the images that have been downloaded to your computer, type:
docker images
The output will look similar to the following:
REPOSITORY TAG IMAGE ID CREATED SIZE
ubuntu latest 1d622ef86b13 3 weeks ago 73.9MB
hello-world latest bf756fb1ae65 4 months ago 13.3kB
As you’ll see later in this tutorial, images that you use to run containers can be modified and used to generate new images, which may then be uploaded (pushed is the technical term) to Docker Hub or other Docker registries.
Let’s look at how to run containers in more detail.
Step 5 — Running a Docker Container
The hello-world container you ran in the previous step is an example of a container that runs and exits after emitting a test message. Containers can be much more useful than that, and they can be interactive. After all, they are similar to virtual machines, only more resource-friendly.
As an example, let’s run a container using the latest image of Ubuntu. The combination of the -i and -t switches gives you interactive shell access into the container:
docker run -it ubuntu
Your command prompt should change to reflect the fact that you’re now working inside the container and should take this form:
root@d9b100f2f636:/#
Note the container id in the command prompt. In this example, it is d9b100f2f636. You’ll need that container ID later to identify the container when you want to remove it.
Now you can run any command inside the container. For example, let’s update the package database inside the container. You don’t need to prefix any command with sudo, because you’re operating inside the container as the root user:
apt update
Then install any application in it. Let’s install Node.js:
apt install nodejs
This installs Node.js in the container from the official Ubuntu repository. When the installation finishes, verify that Node.js is installed:
node -v
You’ll see the version number displayed in your terminal:
v10.19.0
Any changes you make inside the container only apply to that container.
To exit the container, type exit at the prompt.
Let’s look at managing the containers on our system next.
Step 6 — Managing Docker Containers
After using Docker for a while, you’ll have many active (running) and inactive containers on your computer. To view the active ones, use:
docker ps
You will see output similar to the following:
CONTAINER ID IMAGE COMMAND CREATED
In this tutorial, you started two containers; one from the hello-world image and another from the ubuntu image. Both containers are no longer running, but they still exist on your system.
To view all containers — active and inactive, run docker ps with the -a switch:
docker ps -a
You’ll see output similar to this:
1c08a7a0d0e4 ubuntu "/bin/bash" 2 minutes ago Exited (0) 8 seconds ago quizzical_mcnulty
a707221a5f6c hello-world "/hello" 6 minutes ago Exited (0) 6 minutes ago youthful_curie
To view the latest container you created, pass it the -l switch:
docker ps -l
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
1c08a7a0d0e4 ubuntu "/bin/bash" 2 minutes ago Exited (0) 40 seconds ago quizzical_mcnulty
To start a stopped container, use docker start, followed by the container ID or the container’s name. Let’s start the Ubuntu-based container with the ID of 1c08a7a0d0e4:
docker start 1c08a7a0d0e4
The container will start, and you can use docker ps to see its status:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
1c08a7a0d0e4 ubuntu "/bin/bash" 3 minutes ago Up 5 seconds quizzical_mcnulty
To stop a running container, use docker stop, followed by the container ID or name. This time, we’ll use the name that Docker assigned the container, which is quizzical_mcnulty:
docker stop quizzical_mcnulty
Once you’ve decided you no longer need a container anymore, remove it with the docker rm command, again using either the container ID or the name. Use the docker ps -a command to find the container ID or name for the container associated with the hello-world image and remove it.
docker rm youthful_curie
You can start a new container and give it a name using the --name switch. You can also use the --rm switch to create a container that removes itself when it’s stopped. See the docker run help command for more information on these options and others.
Containers can be turned into images which you can use to build new containers. Let’s look at how that works.
Step 7 — Committing Changes in a Container to a Docker Image
When you start up a Docker image, you can create, modify, and delete files just like you can with a virtual machine. The changes that you make will only apply to that container. You can start and stop it, but once you destroy it with the docker rm command, the changes will be lost for good.
This section shows you how to save the state of a container as a new Docker image.
After installing Node.js inside the Ubuntu container, you now have a container running off an image, but the container is different from the image you used to create it. But you might want to reuse this Node.js container as the basis for new images later.
Then commit the changes to a new Docker image instance using the following command.
docker commit -m "What you did to the image" -a "Author Name" container_id repository/new_image_name
The -m switch is for the commit message that helps you and others know what changes you made, while -a is used to specify the author. The container_id is the one you noted earlier in the tutorial when you started the interactive Docker session. Unless you created additional repositories on Docker Hub, the repository is usually your Docker Hub username.
For example, for the user sammy, with the container ID of d9b100f2f636, the command would be:
docker commit -m "added Node.js" -a "sammy" d9b100f2f636 sammy/ubuntu-nodejs
When you commit an image, the new image is saved locally on your computer. Later in this tutorial, you’ll learn how to push an image to a Docker registry like Docker Hub so others can access it.
Listing the Docker images again will show the new image, as well as the old one that it was derived from:
docker images
You’ll see output like this:
REPOSITORY TAG IMAGE ID CREATED SIZE
sammy/ubuntu-nodejs latest 7c1f35226ca6 7 seconds ago 179MB
...
In this example, ubuntu-nodejs is the new image, which was derived from the existing ubuntu image from Docker Hub. The size difference reflects the changes that were made. And in this example, the change was that NodeJS was installed. So next time you need to run a container using Ubuntu with NodeJS pre-installed, you can just use the new image.
You can also build Images from a Dockerfile, which lets you automate the installation of software in a new image. However, that’s outside the scope of this tutorial.
Now let’s share the new image with others so they can create containers from it.
Step 8 — Pushing Docker Images to a Docker Repository
The next logical step after creating a new image from an existing image is to share it with a select few of your friends, the whole world on Docker Hub, or other Docker registry that you have access to. To push an image to Docker Hub or any other Docker registry, you must have an account there.
This section shows you how to push a Docker image to Docker Hub. To learn how to create your own private Docker registry, check out How To Set Up a Private Docker Registry on Ubuntu 18.04.
To push your image, first log into Docker Hub.
docker login -u docker-registry-username
You’ll be prompted to authenticate using your Docker Hub password. If you specified the correct password, authentication should succeed.
Note: If your Docker registry username is different from the local username you used to create the image, you will have to tag your image with your registry username. For the example given in the last step, you would type:
docker tag sammy/ubuntu-nodejs docker-registry-username/ubuntu-nodejs
Then you may push your own image using:
docker push docker-registry-username/docker-image-name
To push the ubuntu-nodejs image to the sammy repository, the command would be:
docker push sammy/ubuntu-nodejs
The process may take some time to complete as it uploads the images, but when completed, the output will look like this:
The push refers to a repository [docker.io/sammy/ubuntu-nodejs]
e3fbbfb44187: Pushed
5f70bf18a086: Pushed
a3b5c80a4eba: Pushed
7f18b442972b: Pushed
3ce512daaf78: Pushed
7aae4540b42d: Pushed
...
After pushing an image to a registry, it should be listed on your account’s dashboard, like that show in the image below.
If a push attempt results in an error of this sort, then you likely did not log in:
The push refers to a repository [docker.io/sammy/ubuntu-nodejs]
e3fbbfb44187: Preparing
5f70bf18a086: Preparing
a3b5c80a4eba: Preparing
7f18b442972b: Preparing
3ce512daaf78: Preparing
7aae4540b42d: Waiting
unauthorized: authentication required
Log in with docker login and repeat the push attempt. Then verify that it exists on your Docker Hub repository page.
You can now use docker pull sammy/ubuntu-nodejs to pull the image to a new machine and use it to run a new container.
Conclusion
In this tutorial you installed Docker, worked with images and containers, and pushed a modified image to Docker Hub. Now that you know the basics, explore the other Docker tutorials in the DigitalOcean Community.
