What is a container image and a registry?

What is a container image and registry

A container image is a lightweight, stand-alone, executable package containing the application code, runtime, libraries, environment variables, and configuration files needed to run the application across various environments. Think of container images as preparing a meal, but instead of buying individual ingredients each time you cook, you use a pre-packaged meal kit that contains all the ingredients, a recipe, and instructions. Whenever you need to cook, you just grab the kit.

Similarly, just as you need to store the meal kits, container registries store, organize, and share container images. In this article, we explore the role of container images and registries in modern software development.

Why are images and registries important?

Container images are portable and read-only. They ensure that an application will work the same no matter where it is used, whether on a developer's local machine or in a cloud-based production environment. This eliminates the "it works on my machine" problem and ensures consistency across each stage of development and deployment.

Teams can store images in a registry to easily share application images and facilitate collaboration, especially in microservices architectures where different teams manage different application parts.

Kubernetes and other orchestration systems can pull the appropriate container images from a registry and deploy them across clusters with minimal overhead.

By using private registries, developers can ensure that only authorized users have access to sensitive images. Additionally, version control through tags allows teams to manage different stages of an application or roll back to previous versions.

How are container images built?

A Dockerfile is typically used to build container images. A Dockerfile is a simple text file that describes how to construct the image. A typical Dockerfile includes the following commands:

• FROM: Specifies the base image, such as a specific version of Ubuntu.
• RUN: Executes commands and is often used to install dependencies.
• COPY: Adds files from the host system into the image.
• CMD: Defines the command that should run when the container starts.

Once the Dockerfile is written, you can use Docker to build the image. The resulting image is stored as a file that can be used to instantiate containers.

To build a Docker image, you can use the docker build command. Here's an example:

docker build -t my-image-name:latest .

Let’s break this example down further:

• docker build: The command to build an image.
• -t my-image-name:latest: The -t flag tags the image with a name (my-image-name) and an optional version tag (latest in this case).
• .: The dot (.) represents the build context. For this example, Docker will look for a Dockerfile in the current directory to use for building the image.

If you have a specific path or a Dockerfile in a different location, you can specify that path instead. For example:

docker build -t my-image-name:latest /path/to/directory

What are registries?

A container registry allows you to store, share, and manage container images in a centralized location for easy distribution. Some registries are public, such as Docker Hub or Google Container Registry, where anyone can upload and download images.

Other registries are for private internal use. Private registries typically require authentication to access, so only authorized users can upload or pull images. Private registries also control access based on user roles or permissions.

Registries are highly available and scalable, enabling developers to quickly retrieve the needed images, even in a production environment with large-scale systems.

Some common container registry options include the following:

• Docker Hub: One of the most widely used public registries, hosting millions of official and community-contributed container images. It allows developers to push and pull images to and from the cloud.
• Google Container Registry (GCR): A private registry provided by Google Cloud. It offers tight integration with Google Cloud services and is ideal for users who want to store their images within the Google Cloud ecosystem.
• Amazon Elastic Container Registry (ECR): A managed container registry service provided by AWS. It integrates with AWS services and provides scalability and security features tailored for AWS users.

How do registries work?

After building a container image, developers push it to a container registry by authenticating with the registry and using a command like docker push to upload the image. Once uploaded, the image is stored in a repository within the registry.

When deploying an application, developers or orchestration systems (such as Kubernetes) pull the required container image from the registry using commands like docker pull or by specifying the image in Kubernetes configurations. The image is then pulled from the registry and used to launch a container.

Within a registry, images are organized into repositories that can contain multiple image versions. Each version is identified by a tag like latest, v1.0, v1.1, etc, to track and manage updates. When a new version of an image is available, developers can pull the updated image by referencing the new tag.