What is Docker Image? A Complete Guide

Docker has revolutionised the Software Development and deployment world by introducing Containerisation technology. Docker Images is at the heart of this technology, which is crucial in encapsulating applications and their dependencies. Docker Images make them portable and easy to manage across different environments.  

In this blog, we will understand the world of Docker Images, their work, and the benefits they bring to the Software Development process. Whether you are a seasoned developer or just a beginner with Docker, this comprehensive blog will help you answer, “What is a Docker Image?” It’ll help you explore its structure, creation process, and advantages.  

Table of Contents 

1) What is Docker?

2) What is a Docker Image?

3) What is a Docker Container?

4) Anatomy of a Docker image

5) Types of Docker Images

6) How Docker Images work?

7) Docker Image commands

8) Benefits of Docker Images

9) Docker container vs. Docker image

10) Conclusion 

What is Docker? 

Before we begin to understand “What is a Docker Image”, let's briefly understand What is Docker. Docker is an open-source platform that helps developers to ensure the deployment of applications inside lightweight, portable Containers. These Containers are isolated from the underlying infrastructure, making them consistent and reliable across various environments. 

What is Docker Image? 

At its core, a Docker Image is a lightweight, standalone, and executable software that includes everything needed to run code, libraries, environment variables, and system tools. Docker Images serve as the blueprint for creating Docker containers, allowing developers to package their applications along with all dependencies into a single, standardised unit.  

Docker Images primarily serve as the building blocks of Containers, encapsulating everything an application needs to run, including code, runtime, libraries, dependencies, and system tools. Think of a Docker Image as a snapshot or a blueprint of a complete application environment, compactly packaged and ready to be replicated and executed anywhere. 

Understand how Agile and DevOps band together to create a secure environment with Certified Agile DevOps Professional (CADOP)! 

What is a Docker Container?

Often, amateurs in the Contenarisation domain confuse Docker Image with Docker Containers. So, What is Docker Container?  A Docker Container is a standard unit of software that packages an application and its dependencies, ensuring it runs seamlessly across different computing environments. It's a lightweight, executable software bundle that includes everything—code, runtime, libraries, tools, and settings—required to run an application. Docker Containers operate within their own isolated environment, offering consistency and portability. They function uniformly across various platforms, from development to testing and deployment, making them a practical solution for software developers to streamline workflows and ensure applications work consistently across diverse environments.

Anatomy of a Docker Image

A Docker Image comprises multiple layers that encapsulate the configuration for a Container environment, consisting of system tools, libraries, dependencies, and files. These layers include:

1) Base image: The build command can construct This initial layer from scratch.

2) Parent image: Alternatively, a reused parent image can be the first layer in a Docker Image, forming a base for subsequent layers.

3) Layers: Added to the base image, layers are code configurations facilitating Container functionality. They can be viewed using Docker commands, showcasing the top-layer image, its details, and file sizes, with intermediate layers cached for easier viewing. Docker employs storage drives to manage layer contents.

4) Container layer: Beyond creating a new Container, a Docker Image generates a writable layer to record modifications within the running container, storing new, deleted, and altered files.

5) Docker manifest: This file within the image, in JSON format, describes the image contents, encompassing tags and digital signature information.

Types of Docker Images

Docker Images serve as self-contained units that encapsulate an application along with its runtime environment and dependencies, making them easily portable and deployable across different systems. Below are some common examples to understand What is a Docker Image and what its types are. These examples shall also demonstrate the versatility and practicality of this Containerisation technology:

NGINX Web Server Image 

The NGINX Web Server Image is one of the most popular and widely used Docker Images. It allows developers to quickly set up a lightweight and efficient web server. It can also serve static content or act as a reverse proxy for dynamic web applications. With this image, deploying a web server becomes a breeze, enabling seamless scaling and load balancing of web traffic. 

With the NGINX Docker Image, developers can effortlessly set up a lightweight and high-performance web server, making it ideal for hosting websites and handling incoming HTTP requests efficiently. 

MySQL Database Image  

Docker provides a MySQL Database Image that simplifies the process of deploying a relational database. Developers can create a MySQL container with just a few commands, and the image comes pre-configured with default settings. This means you can have a fully functional MySQL Database up and running in a matter of seconds.  

It is ideal for development, testing, or small-scale production scenarios. Developers can quickly create MySQL Containers with pre-configured settings, making it easy to set up local development environments or run databases for small-scale applications. 

Elasticsearch and Kibana Image  

The Elasticsearch and Kibana Image combo provides a powerful solution for developers and data analysts dealing with large volumes of data. Elasticsearch is a highly scalable and diversified search engine, while Kibana offers a web interface for data visualisation and exploration. With Docker Images for both Elasticsearch and Kibana, setting up a complete data analytics environment becomes swift and hassle-free.  

WordPress with MySQL Image 

Docker facilitates the bundling of multiple services into a single Container. An excellent example is WordPress with MySQL Image, which combines the WordPress Content Management System (CMS) with the MySQL Database in a single Container. This pre-configured image streamlines the setup process for running a WordPress website, ensuring seamless communication between the application and the database. 

This combination allows developers to streamline the website development process and acts as a bridge between the database and the application. It enables developers to not only create a dynamic interface, but also manage pieces of data that will be entered via the freshly created website’s input fields. 

TensorFlow Image  

For Machine Learning enthusiasts, Docker provides a TensorFlow Image, making it easy to set up a development environment for building and deploying Machine Learning models. TensorFlow is a popular open-source Machine Learning library, and the Docker Image ensures that all required dependencies and configurations are readily available. 

Jenkins Continuous Integration Image  

Docker Images extend their benefits to the world of Continuous Integration or Deployment (CI/CD). The Jenkins Image offers a powerful automation server that facilitates building, testing, and deploying applications. With this image, developers can quickly set up Jenkins instances and automate their software delivery pipelines.  

The Jenkins application image is a go-to choice for developers working with Node.js-based applications. This image includes the runtime and allows developers to package, deploy, and run Node.js applications within isolated Containers. 

Redis in-memory Database Image 

Redis is a fast and versatile in-memory data store commonly used for caching, session storage, and queuing. Docker simplifies Redis deployment, allowing developers to run isolated Redis instances with ease. This image is particularly useful in distributed systems and microservices architectures. It is a robust, lightweight, and powerful web framework. Using Docker with Flask simplifies application deployment and ensures consistency across various environments. 

Ubuntu Base Image 

Docker provides a wide range of base images for different operating systems. The Ubuntu Base Image is a common choice for developers who need a familiar Linux environment to build and run their applications. From this Base, developers can layer additional components and configurations to create custom images tailored to their specific needs. 

These types of applications serve the extensive range of Docker Images available on Docker Hub and other container registries. Docker's flexibility and thriving community of contributors continuously expand the available images, covering an extensive spectrum of applications and services. The adoption of Docker Images has revolutionised the way developers work with applications, enabling them to deploy, scale, and manage software with unmatched ease and efficiency. 

Anticipate attacks, refine strategies, and engage the right DevOps practices with our SAFe DevOps Certification Training.

How do Docker Images work?

The process of creating a Docker Image starts with the development of a "Dockerfile." A Dockerfile is nothing but a text file that contains a set of instructions to specify the application's dependencies, configuration, and other essential components. These instructions are executed step by step to build the final Docker Image. The beauty of Dockerfiles lies in their simplicity and reproducibility, allowing developers to automate the image-building process and easily share their work with others. Here’s how Docker Images work:

Layers and Union File System

Docker Images are built on the concept of Layers. Each instruction in the Dockerfile (a script used to create an image) adds a new Layer to the image. These Layers are cached, making subsequent builds faster. The Union File System merges these Layers into a single cohesive view.

Building Docker Images

To create a Docker Image, developers use a Dockerfile, which contains all the instructions for building the image. The Dockerfile specifies the base image, environment variables, application code, and other dependencies required for the application.

Pulling and pushing Images to registries

Docker Images are typically stored in container registries like Docker Hub. Developers can pull these images to their local environment for development and testing. Once an image is ready for production, it can be pushed to the registry for deployment.

Docker Image commands

Docker offers a range of primary image commands, which are further categorised into child commands. Here are some of these commands:

1) docker image build: Constructs an image from a Dockerfile.

2) docker image inspect: Provides information on one or more images.

3) docker image load: Loads an image from a tar archive or streaming input (STDIN).

4) docker image prune: Deletes unused images.

5) docker image pull: Retrieves an image or repository from a registry.

6) docker image push: Sends an image or repository to a registry.

7) docker image rm: Deletes one or more images.

8) docker image save: Archives one or more images to a tar format (default streaming to STDOUT).

9) docker image tag: Creates a reference tag (TARGET_IMAGE) for the SOURCE_IMAGE.

Additionally, Docker Image commands are utilised to customise Docker Images, with examples including:

1) docker image history: Displays an image's history, outlining modifications and layers.

2) docker update: Facilitates the update of container configurations.

3) docker tag: Establishes tags like TARGET_IMAGE for organising container images.

4) docker search: Searches Docker Hub for specific requirements.

5) docker save: Allows users to save images into an archive.

6) docker compose: Used for managing environment variables.

Defend your valuable process frameworks with the best DevOps practices through the Certified DevOps Security Professional Course.

Benefits of Docker Images 

The portability of Docker Images is one of their key advantages. Once an application is packaged into a Docker Image, it becomes agnostic to the underlying infrastructure and operating system.  

This portability enables seamless movement of applications between different environments, making it easy to deploy and scale applications without worrying about compatibility issues. Working with Docker Images offers several benefits that streamline the software development and deployment process. Some of the key benefits include:

Portability and consistency 

Docker Images encapsulate the application and its dependencies, ensuring consistency across development, testing, and production environments. This portability eliminates the common "it works on my machine" problem and simplifies the deployment process. 

Docker Images are designed for reusability. When building a new image, Docker can reuse layers from existing images that share similar dependencies, saving both disk space and build time. 

Efficient resource utilisation 

When changes are made to a Docker Image, only the affected layers need to be rebuilt. Since the unaffected layers remain cached, this optimises the build process and reduces the time required for rebuilding images. 

Since Docker containers share the host OS kernel, they are incredibly lightweight and consume fewer resources compared to traditional virtual machines. This efficient utilisation of resources allows you to run more containers on the same infrastructure. 

Control and collaboration 

Docker Images can be versioned, making it easy to fall back to previous versions if needed. Moreover, these images can be easily shared and distributed among team members, promoting collaboration and reducing conflicts between development and operations teams. 

The Union File System merges the individual Layers into a single cohesive view, making it appear as if all the Layers are merged into one. This approach allows for space-efficient storage of images while still providing isolation and separation between layers. 

Additionally, Docker Images can be easily distributed and shared with others. By sharing the Dockerfile and its Layers, developers can ensure that their colleagues or team members have a consistent and reproducible environment for running the application.

Docker container vs. Docker Image

A Docker container serves as a virtualised runtime space for application development, enabling the creation, operation, and deployment of applications in an isolated environment detached from the underlying hardware. Docker containers can virtually operate multiple remote processes by leveraging a single machine and sharing its kernel. This approach results in containers being notably lightweight.

On the other hand, a Docker Image represents a snapshot of a Docker container at a specific moment. These images are immutable, meaning they can't be altered but can be easily duplicated, shared, or removed. This characteristic is beneficial for testing new software or configurations as it preserves the image in its original state regardless of any changes.

A runnable image is essential for containers to function, as containers rely on these images to establish runtime environments necessary for running applications.

Conclusion 

Docker Images serve as a vital building block of Docker's containerisation technology. Docker Images provide efficiency, reusability, and easy distribution by using a Layered approach and the Union File System. Now that you have understood What is a Docker Image and how Docker Images empower developers to create portable and scalable environments for their applications. It’ll now become a lot easier for you to deploy containers and use Docker Images.

Register now for our Continuous Integration Training with TeamCity and learn how to build dynamic applications!