Container Image

What is a container image?

A container image is like a template for a software application. It stores all the procedures for running the application in a cloud environment. 

The container image makes sure the software works similarly in all the systems and servers with a familiar container. It provides consistent usage, making deployment quick and easy. 

With this process, engineering teams save on cost by efficiently using resources and reducing maintenance frequency. Many organizations use container registry software to store container images, helping teams manage images distributed throughout applications and networks. 

Types of container images

Different images serve different purposes while using the container. Below are some of its common types.

• Base images serve as the starting point for creating new container images. They generally contain only the operating system.
• Official images are provided and maintained by the software vendor. 
• Custom images help meet specific requirements. They start with a base or official image and add layers with customized code and configurations.
• Application-specific images include both the application and its dependencies. They’re tailored for a particular application. 
• Language-specific images possess the container engine for a specific programming language, like Python, Node.js, or Java.
• Service images comply with special requirements, such as a database, web server, or message queue.
• Minimal images contain only the essentials required to run an application. They help reduce size and potential security risks.
• Multi-stage images can serve a specific purpose – like building code or running tests – at each step of the process. The last stage contains only the necessities to run the application.
• Layered images allow better use of cache and storage as built-in layers.

Core features of container images

Container images are critical in the development and deployment of cloud-native applications. Here are some notable features that make it a suitable choice for engineering teams. 

• Immutable. Container images are non-changeable and maintain consistency across various environments.
• Lightweight. They contain only necessary components instead of the entire operating system, making them lightweight. 
• Portable. Any platform or cloud that supports container runtime can use container images.
• Versioning. Container images keep previous versions accessible if needed.
• Layered file system. Each instruction in the Dockerfile adds a new layer when building an image. This approach ensures optimal storage as common layers between images are only stored once.
• Repository storage. A container registry or repository stores container images with proper versioning.
• Security scanning. Some container registries offer security scanning features to detect vulnerabilities.
• Metadata. IT professionals can add metadata to container images to provide additional information about the image.

How does a container image work?

Here's a step-by-step explanation of how a container image works. 

• Creation. A text file called a "Dockerfile" or similar creates a container image, based on the container platform. This file has instructions for what should go in the image, like the base operating system, application code, and other files or settings.
• Build. The container engine processes the Dockerfile to build the actual image. The image is made layer by layer, following the instructions in the Dockerfile.
• Storage. The container registry stores the image. It can be a public or a private registry.
• Deployment. The developer pulls the image, either to their local machine or a server, to run it. 
• Run. The container engine reads the image and creates an isolated container to run the application. This container has its own file system, networking, and isolated process space, making it independent of the host system.
• Runtime. While running, the container uses the libraries and dependencies from its image to execute the application. It can be stopped, started, and removed independently of other containers.
• Scale. If needed, multiple containers can run from the same image, making it easy to scale the application.
• Update. If the application or its dependencies need an update, a new image is created, and the existing containers from the old image replace the new containers. 

Container image vs. docker container

A container image is a blueprint with all codes and dependencies required for an application to run in a cloud-native environment. It’s immutable to ensure consistent behavior of the application when run on various platforms or clouds.

Docker containers are self-contained runtime instances. They comprise a read-only layer and add a read-write layer on the top. 

Learn more about container registry and why it’s a one-stop solution for building cloud-native applications.