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What is Docker?

Docker is a system that allows you to containerize a deployed application along with all of its configuration settings and associated files. When you deploy a Docker container image, it is ready to run. If you mess up a container, you can destroy it and redeploy the image.

Notes from a workshop by Nick Stayer on building and running a Dockerfile (2019-04-26)

Docker Compose is a feature of Docker that allows several containers to interact with each other in a pre-determined way. It allows one to archive a complex setup of several applications in a way that others can easily deply them.

Introduction to Docker Compose

Docker Machine is a system for managing Docker containers on several servers. This can include remote servers (such as those on Amazon Web Services or Digital Ocean), as well as a localhost server on your own computer. You can switch between different servers, then interact with them in the same way (through Docker or Docker Compose commands) as you would with the default localhost server using generic Docker.

Introduction to Docker Machine

Setting up and testing Docker

Note: these instructions cover installing and testing Docker using preexisting container images. They do NOT cover creating your own Docker container images.

Important note for Windows users

If you are a Windows user, there are several things you need to consider before attempting to install the Docker desktop client that runs the show when you are using docker:

If Virtualization is disabled (see above), you will need to make a change to your BIOS to enable it. This is not for amateurs, so either get professional help or use a different computer.

If you meet these three requirements, then proceed.

Docker concepts

This minimal non-technical introduction is designed for casual users. For a more complete introduction, see the Docker Overview from the Docker online documentation. There are also Get Started pages, although they go into detail about creating containers.

A container is a self-contained system that includes everthing necessary to run an application on your computer. It’s similar to a virtual machine, except that it is not generic and includes only the components necessary to run the application. Once you have deployed a container on your computer, you can start and stop it. When it runs, it can change as you interact with it. When you stop it, its state is “frozen” until the next time you start it up again. It is possible to move the container elsewhere with its state preserved, or to delete it. You can also deploy several containers of the same application at the same time, and use them for different purposes.

An image as like a frozen, read-only image of a container that includes all of the information necessary to set it up as a running container. It is possible to create your own container images, but most users will start with a container set up by someone else. The most common place to get Docker images is Docker Hub. Docker Hub is sort of like GitHub, but specifically for Docker images.

The Docker client application is software that you install on your computer that enables you to pull images from Docker Hub and install them as containers, and to manage (start, stop, track, and delete) the containers that you’ve deployed on your computer.

Docker Compose is a Docker tool for running applications that consist of multiple containers. It makes use of information that tells how the containers should interact with each other.

Installing and testing docker

Installing the Docker client application

In order to download the Docker client, you first need to create a Docker account by going to https://id.docker.com/.

If you are just getting started with Docker, you will probably want to install the free Community Edition. Go to the Docker CE page and find the version for your operating system. The install instructions are fairly straightforward and won’t be repeated here.

Once you’ve installed the Docker client, it will start up automatically when you boot your computer and run in the background. You can know it’s there by looking in the system tray in the lower right of your screen on PC, or the menu bar in the upper right of your screen on Mac. You should see the little whale icon there. Clicking on it brings up options for managing the application.

Testing the Docker client

In order to use the Docker client, you need to issue commands using your computers command line console. If you haven’t used the console before, here are instructions for getting to it on PC and Mac:

Windows In Windows, the application for the command line is called “Command Prompt”. The easiest way to get to the command prompt is to start typing “command” in the search box next to the start button. When Command Prompt shows up in the results, click on it to open a Command Prompt window.

When you enter the Command Prompt window, you should see a line with the path to your user directory, followed by a “>” character. This is the system prompt. It means that you can issue any kind of command line command that Windows will understand.

Mac On a Mac, the application for the command line is called “Terminal”. The easiest way to get to the command line via Terminal is to click on the Spotlight Search icon (small magnifying glass in the upper right of the screen) and start typing “terminal” in the search box. When terminal.app shows up in the results, click on it to open a Terminal window.

When you enter the Terminal window, you should see a line with the your computer name, a tilde (~) followed by your username, and finally a “$” character. This is the system prompt. It means that you can issue any kind of command line command that the Mac operating system will understand.

Note: The Mac operating system is build on the Linux operating system. So the commands that you give in this window are sometimes called “bash commands” (a type of Linux commands). Hence you see “bash” listed in the header of the terminal window.

Running some tests

At the command line, enter

docker info

If Docker is running, you should get some information. If you get a message like “Command not found” or “‘docker’ is not recognized as an internal or external command, operable program or batch file.”, then either the installation failed or Docker is not running. In this case, you will need to get troubleshooting help from someone.

To see what images have been downloaded to your computer, enter

docker image ls

and to see what containers have been deployed on your computer, enter

docker container ls -all

This command will show containers that are currently running as well as ones that have been run in the past, but have been stopped. If you run these two commands now, you may not see anything listed.

To try actually running an image, enter

docker run hello-world

The Docker client will pull the “hello-world” image from Docker Hub to your computer, then create a “hello-world” container from that image and run it. All that this particular application does is to print some text on your screen, which you can read after it runs. Now if you repeat the image list command shown above (easily retrieved at the command line by pressing the up arrow several times, then pressing Enter/Return), you will see the hello-world image listed. Listing the containers will show that the hello-world container is no longer running.

Cheat sheet

Here are some of the most important Docker commands:

# pull image from Docker Hub; "repo" is repository name, "image" is image name:
docker pull repo/image

# to run the container named "image" from the "repo" repository, version 2.1.5 for the first time, associating it with the shortened name "shortName":
docker run --name shortName repo/image:2.1.5

# to stop the container with the shortened name "shortName":
docker container stop shortName

# to restart the container using the shortened name "shortName":
docker restart shortName

# to list all Docker containers:
docker container ls -all
# leave the "-all" off to see only containers that are currently running

# to remove a container using the ID shown in the previous command:
docker container rm {containerId}
# replace {containerId} with the actual container ID

# to list Docker images that have been pulled:
docker image ls

# to remove an image using the ID shown in the previous command:
docker image rm {imageId}
# replace {imageId} with the actual image ID

If you really want to get rid of everything and start over, see these instructions

Things to try

Example using Docker to install Blazegraph locally

Example using Docker Compose to install the components of Wikibase locally

Revised 2020-01-07

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License: CC BY 4.0.
Credit: "Vanderbilt Libraries Digital Lab - www.library.vanderbilt.edu"