When you think of Docker images, what comes to mind? For most people, it’s probably something like an image of the famous blue whale. Or the image of a red pandas. Or perhaps an image of some weird creature which you will never see in real life. If that sounds about right, then you’ve stumbled on the right article. Otherwise, we wonder what search engine did you use to find this article? Anyway…
What is a Docker image? #
The image is the first thing that comes to mind when we start talking about Docker. If we’re using a simple definition, an image is a virtual machine in Docker. It’s a single file that holds the exact instructions required to build an instance of a running system. This file can then be reused to create many different instances – each called a container. The Docker image is the definition of the instructions and the process to build a container. In other words, it is a set of instructions that are required to build a container. Docker images are the basis of containers. Containers are created from images, and many containers can be built from one image. Images are also the source of any container’s persistent data.
Docker image benefits #
Build once, deploy many times – Once you have published an image to a registry and have it available for download, you can use that image to create containers many times. If you have some image with a standard operating system, for example, then you can use it to create containers that are ready to deploy web servers, database servers, and anything else you might need.
Save time – Docker images can significantly reduce the time it takes to build a new environment. You can use an existing image to create a new container that is pre-configured, which means you can skip all the steps that are usually required to create a whole new environment.
Standardise management – Building a standard image, with all the required software, configuration files, and data, can significantly improve the way your team manages new environments.
Share code – A Docker image is just like a virtual machine that you can share with your team.
Inexpensive – Installing applications and services on a virtual machine can be time consuming and expensive. Containers make it possible to get started with just one click and without the need for virtual machines. With Docker, you can run your applications on containers that are ready to go without the need to install anything on your computer first.
Risk reduction – Containers are disposable. If you have a faulty application or an application whose data is not trustworthy, you can simply stop the container. In addition, containers can be restarted if they fail.
How to create a Docker image? #
There are two steps you need to follow to create a Docker image. First, you need to create a Dockerfile, which is a file that contains the instructions that are required to create the image. Then, you need to build the image with the docker build command.
The different types of Docker images #
There are two types of Docker images: BASE and DIFF. A BASE image contains only the operating system and packages that are required to run Docker. A DIFF image contains everything from a base image, plus any extra applications, configuration files, and data.
Image size and layer manipulation #
The size of an image is an important consideration. A smaller image will have fewer dependencies and will be quicker to build. It will also take less time to transfer and will consume less disk space on the Docker host. You can reduce the size of your image by removing unnecessary layers. A layer is a part of an image that is saved to a file during the build process. By default, the number of layers is three. You can override the default by setting the COPY command in your Dockerfile. You can also manipulate the layers that are part of your image. You can use the –squash flag with the docker build command to combine two or more layers into a single layer. Or, you can use the –overlay flag to add one layer on top of another.
Image Registry and registries API #
A registry is an image repository that stores images and makes them available for download to anyone who has access to the registry. The Docker Registry is the default image repository for Docker. The Registry ships with several APIs that make it easy to manage images. You can add, remove, and search for images, as well as create, delete, and manage access to repositories.
Concluding Words #
Once you learn about Docker images and how to build them, you can quickly create containers with your images. Docker images are a great way to share code, save time, and create standardised environments. However, you should keep in mind that images can take a long time to build if they include too many dependencies.
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