How Do Docker Images Work?

Introduction

In this article, we will be exploring the various components that make up a Docker image and how they work together to create containers. We will start by looking at image layers and how they are used to store changes made to an image.

Anatomy of a Docker Image

A Docker image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software, including the application code, libraries, dependencies, and runtime. It is a blueprint for creating Docker containers, which are executable instances of Docker images.

• Docker images are built from a series of layers, each representing a separate instruction in the image's Dockerfile. A Dockerfile is a text file that contains the instructions for building an image. It specifies the base image to use, any dependencies that need to be installed, and any commands or scripts that need to be run to set up the image.

• Each time a command is run in a Dockerfile, a new layer is created on top of the previous layers. This allows Docker images to be built incrementally, with each layer representing a separate instruction in the Dockerfile. It also allows Docker to reuse layers between images, which can help reduce the size of images and improve build times.

• Docker images are stored in a registry, which is a centralized location for storing and distributing Docker images. When you run a Docker container, Docker pulls the image from the registry and runs it on your local machine. Several public registries are available, such as Docker Hub, the default registry for Docker. You can also set up your private registry if you want to store and manage images internally.

Image Layers

In Docker, an image layer is a change made to a Docker image. When you build a Docker image, you specify a series of instructions in a Dockerfile. Each instruction in the Dockerfile creates a new layer in the image.

For example, if you specify a command to install a package in your Dockerfile, a new layer will be created that includes the package and its dependencies. If you make another change, such as adding a file to the image, another layer will be created on top of the previous one.

One of the benefits of using image layers is that they allow you to roll back changes to an image. If you make a change to an image and later realize that it caused problems, you can use a previous layer to revert the image to a previous state.

Container Layers

In Docker, a container layer is a change made to a running Docker container. When you run a Docker container, it is based on a Docker image. Any changes you make to the container, such as installing a package or modifying a file, are stored in a separate layer on top of the base image.

Like image layers, container layers are stored in a union file system, which allows Docker to combine the base image layers with the container's layers into a single view of the file system. This allows Docker to isolate the changes made to the container from the base image and other containers while still allowing the container to access the files and resources it needs to run.

One of the benefits of using container layers is that they allow you to easily roll back changes to a container. If you make a change to a container `and later realize that it caused problems, you can use a previous container layer to revert the container to a previous state.

Parent Image

In Docker, a parent image is an image that is used as the starting point for building a new image. When you create a Docker image, you can specify a base image to use as the starting point for your image. This base image is known as the parent image.

For example, if you want to create a Docker image for a web application that runs on Apache, you might start with a base image that includes the Apache web server and its dependencies. This base image would be the parent image for your web application image.

The parent image serves as the foundation for your image, providing the base operating system and any necessary libraries and dependencies.

Base Image

In Docker, a base image is an image that serves as the foundation for building new images. A base image typically includes the base operating system and any necessary libraries and dependencies but does not include any application-specific code or configuration.

Many base images are available in public repositories, such as Docker Hub, that you can use as a starting point for your images. You can also create your base images if you need a specific operating system or set of dependencies that are not available in a public repository.

Docker Manifest

Docker manifests are a way to define a set of images that are related to each other and to specify the dependencies between them. They allow you to define a logical grouping of images so that you can easily reference them as a single entity.

A manifest is a JSON file containing information about a set of images, including their names, dependencies, and the platforms for which they are intended. The manifest also specifies the order in which the images should be built, which can be useful when building complex, multi-image applications.