What is Docker Image?

By | October 9, 2021

Docker is an open platform used to develop, ship, and run applications. With Docker, you can separate the applications from the infrastructure to quickly deliver your software.

With the help of Docker’s methodologies, you can save plenty of time between writing a code and running it in production. There are ample elements associated with Docker, and one such vital part is a Docker image.

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A Docker image is a read-only template that inhabits instructions to create a container that runs on the Docker platform. It further encapsulates applications and pre-configured server environments that you can use privately or share with other Docker users.

If you are a new Docker user, this article will help you to understand the concept of Docker images in detail. Before we begin, here is the agenda:

  • What is Docker Image?
  • Anatomy of Docker images
  • Docker image repositories
  • Docker image use cases
  • The process to create a Docker image

Let’s get started!

What is a Docker Image?

Whenever you execute code in the Docker container, you need a Docker image. Docker images are basically an instruction set to build a Docker container. More precisely, they are like templates.

As you know, developers use Docker to create and run several applications in containers. Whereas, a Docker image specifically inhabits libraries, tools, application code, dependencies, and several other files in order to make the application run.

In the case of Docker images, each one of them is built with multiple layers. Every layer is unique and originates from the previous layer.

Now, how do these layers help? Or what is the role of these layers?

Well, these image layers are read-only and help to enhance the speed of Docker builds. They further improve the reusability and also reduce disk use.

If you establish a container, a layer adds up on the images that can not be changed.

Now, let us understand the anatomy of a Docker image.

Anatomy of Docker Images

As you just understood that each image has several layers, and every layer inhabits all the dependencies to configure a container environment. Here is the bifurcation of each part of an image:

  • Base image: With the help of the build command, you can create the first layer from the beginning.
  • Parent image: A parent image can also be the first layer of the Docker image. This acts as the foundation for all layers and is a reusable image.
  • Container layer: The container layer can be used to customize containers. A Docker image creates a writable container layer alongside the new container. If there are any changes made in the running container, then this layer hosts those changes and later acts as a storage for the newly written or deleted files as well.
  • Layers: In the base image, several layers are added by using the code that enables them to run in a container. You can view every layer of a Docker image through the Docker history command in the CLI or under /var/lib/docker/aufs/diff. Docker has several storage drives that help to manage image layer contents.
  • Docker manifest: This is an additional file that uses JSON format to explain an image by using digital signatures or image tags.

Besides, the disk space in containers and Docker images can be a tad ambiguous. Therefore, it’s essential to understand the difference between virtual size and size. Here is a brisk explanation of these terms:

  • Virtual Size: Virtual size refers to the space that the disk occupies for the writable layer and the container.
  • Size: Size refers to the space that the disk occupies for the container.

Moreover, there are several Docker image repositories that we will discuss in the next section.

Docker Image Repositories

All the Docker images are stored in either public or private repositories, such as the Docker hub cloud registry service. From these repositories, users are able to test as well as share images, and also deploy containers. The Docker hub cloud registry also helps to handle image management.

Users can create new images from the older ones. Moreover, you can use the Docker push command to add as many custom images as you want to the Docker hub.

Docker also has the feature to provide feedback to the authors before publishing any image. As soon as the image is published the author can make updates as and when required. Authors also need to stay alert at the time of sourcing an image as there can be a malicious attack from outside. Usually, a hacker may get access to the system and the user may use corrupted images.

Additionally, there is one more concept known as the latest image. All the darker images that have ‘latest’ tag, simply mean it’s a default tag. Many misinterpret them as the most recently pushed version of an image.

After understanding the basic concepts of the Docker image, let us look at some of its use cases.

Docker Image Use Cases

To run a containerized application, you need a number of dependencies. For instance, environment variables, runtimes, libraries, config files, codes, etc. Whenever you attach an image to a Docker environment, you can run it as a Docker container.

Moreover, Docker images save a lot of time, and the developers or the users do not have to build every image from scratch. The reason is that Docker images can be deployed on any host and are reusable. Hence, the developers can easily use static image layers in several projects.

Now, let us explore the procedure to create a Docker image in detail.

Process to Create a Docker Image

There are two methods to create a Docker image, namely interactive and Dockerfile method. Both the methods are discussed as follows:

  • Interactive method

The interactive method is known as the easiest way to build Docker images.

If you use this method, you can run a container from an already existing Docker image and make several changes manually to the environment.

The first step in this method is to launch Docker and switch to the terminal session. After that, use the Docker run command to start the shell session from the container launched from the image. Do not forget to use the tag name, otherwise, Docker takes the latest version of the image.

  • Dockerfile method

Dockerfile helps to make specifications to create an image. It can be a time-consuming and complicated process but it is a reliable one. In this method, you have to create a Docker file and add commands required by the image.

As soon as the Dockerfile starts, you need to use the .dockerignore files to exclude files that are not required for the final build. This file is located in the root directory. Then, in order to create a Docker image, the Docker build command is used. In the end, you can use the Docker images command to check the images.

As we proceed, you must understand some essential Docker image commands that are used frequently.

There are several categories of Docker images commands. One of them is known as child commands. Here is a list:

  • docker image inspect: It showcases information on multiple images.
  • docker image build: It helps you to create an image from a dockerfile.
  • docker image prune: It removes any unused image.
  • docker image load: You can load any image from tar archives to receive or read inputs.
  • docker image push: With the help of this command you can easily push any image or repository into a registry.
  • docker image pull: You can pull any image or a repository out from a registry.
  • docker image rm: It removes multiple images.
  • docker image save: You can save multiple images to a tar archive.
  • docker image tag: It helps you to create a tag known as target_image that points to source_image.

Moreover, to customize the Docker images, use the following commands:

  • docker update: It helps you to update the container configuration.
  • docker image history: Use this command to know the history and any changes made to the layers.
  • docker search: It searches the Docker hub.
  • docker tag: This command helps users to organize and group container images.
  • docker compose: This helps to tackle an environment variable.

So, these were some of the most used commands while using Docker images.

Conclusion

Docker images play an essential role in the Docker world. They help you to create applications in multiple containerized environments.

These images are packed with tools, libraries, code, and several other essential files needed for an application to run successfully.

We hope that this article helped you understand the concept of Docker images in detail.

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