How to Create a Docker Image

In the previous article, we learned about how to get started with Docker on Linux, macOS, and Windows. In this article, we will get a basic understanding of creating Docker images. There are prebuilt images available on DockerHub that you can use for your own project, and you can publish your own image there.

We are going to use prebuilt images to get the base Linux subsystem, as it’s a lot of work to build one from scratch. You can get Alpine (the official distro used by Docker Editions), Ubuntu, BusyBox, or scratch. In this example, I will use Ubuntu.

Before we start building our images, let’s “containerize” them! By this I just mean creating directories for all of your Docker images so that you can maintain different projects and stages isolated from each other.

$ mkdir dockerprojectscd dockerprojects

Now create a Dockerfile inside the dockerprojects directory using your favorite text editor; I prefer nano, which is also easy for new users.

$ nano Dockerfile

And add this line:

FROM Ubuntu

Save it with Ctrl+Exit then Y.

Now create your new image and provide it with a name (run these commands within the same directory):

$ docker build -t dockp .

(Note the dot at the end of the command.) This should build successfully, so you’ll see:

Sending build context to Docker daemon 2.048kBStep 1/1 : FROM ubuntu---> 2a4cca5ac898Successfully built 2a4cca5ac898Successfully tagged dockp:latest

It’s time to run and test your image:

$ docker run -it Ubuntu

You should see root prompt:

[email protected]:/# 

This means you are literally running bare minimal Ubuntu inside Linux, Windows, or macOS. You can run all native Ubuntu commands and CLI utilities.

vpZ8ts9oq3uk--z4n6KP3DD3uD_P4EpG7fX06MC3

Let’s check all the Docker images you have in your directory:

$docker imagesREPOSITORY TAG IMAGE ID CREATED SIZEdockp latest 2a4cca5ac898 1 hour ago 111MBubuntu latest 2a4cca5ac898 1 hour ago 111MBhello-world latest f2a91732366c 8 weeks ago 1.85kB

You can see all three images: dockp, Ubuntu, andhello-world, which I created a few weeks ago when working on the previous articles of this series. Building a whole LAMP stack can be challenging, so we are going create a simple Apache server image with Dockerfile.

Dockerfile is basically a set of instructions to install all the needed packages, configure, and copy files. In this case, it’s Apache and Nginx.

You may also want to create an account on DockerHub and log into your account before building images, in case you are pulling something from DockerHub. To log into DockerHub from the command line, just run:

$ docker login

Enter your username and password and you are logged in.

Next, create a directory for Apache inside the dockerproject:

$ mkdir apache

Create a Dockerfile inside Apache folder:

$ nano Dockerfile

And paste these lines:

FROM ubuntuMAINTAINER Kimbro Staken version: 0.1RUN apt-get update && apt-get install -y apache2 && apt-get clean && rm -rf /var/lib/apt/lists/*ENV APACHE_RUN_USER www-dataENV APACHE_RUN_GROUP www-dataENV APACHE_LOG_DIR /var/log/apache2EXPOSE 80CMD ["/usr/sbin/apache2", "-D", "FOREGROUND"] 

Then, build the image:

docker build -t apache .

(Note the dot after a space at the end.)

It will take some time, then you should see successful build like this:

Successfully built e7083fd898c7Successfully tagged ng:latestSwapnil:apache swapnil$

Now let’s run the server:

$ docker run –d apachea189a4db0f7c245dd6c934ef7164f3ddde09e1f3018b5b90350df8be85c8dc98

Eureka. Your container image is running. Check all the running containers:

$ docker psCONTAINER ID IMAGE COMMAND CREATED a189a4db0f7 apache "/usr/sbin/apache2ctl" 10 seconds ago

You can kill the container with the docker killcommand:

$docker kill a189a4db0f7

So, you see the “image” itself is persistent that stays in your directory, but the container runs and goes away. Now you can create as many images as you want and spin and nuke as many containers as you need from those images.

That’s how to create an image and run containers.

To learn more, you can open your web browser and check out the documentation about how to build more complicated Docker images like the whole LAMP stack. Here is a Dockerfile file for you to play with. In the next article, I’ll show how to push images to DockerHub.

Learn more about Linux through the free “Introduction to Linux” course from The Linux Foundation and edX.

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