This blog post details how to build a container image from scratch without using Docker or other containerization tools. It explains the core components of a container image: a root filesystem with necessary binaries and libraries, metadata in a configuration file (config.json), and a manifest file linking the configuration to the layers comprising the root filesystem. The post walks through creating a minimal root filesystem using tar, creating the necessary configuration and manifest JSON files, and finally assembling them into a valid OCI image using the oci-image-tool utility. This process demonstrates the underlying structure and mechanics of container images, providing a deeper understanding of how they function.
This blog post, "Build a Container Image from Scratch," provides a comprehensive walkthrough of constructing a container image without relying on tools like Docker. The author meticulously details the process, emphasizing the underlying mechanisms and the core components involved. The tutorial begins with a foundational explanation of container images, highlighting their role as self-contained packages encompassing all necessary dependencies for running an application. This includes not only the application's code itself but also system libraries, tools, and configuration files. The core of the image construction revolves around creating a root filesystem. This is accomplished by employing a temporary directory where the essential directory structure mimicking a Linux filesystem is established. Crucial directories such as /bin, /lib, /dev, and others are populated with the requisite files.
The post then meticulously guides the reader through the process of copying a statically compiled "hello world" program, written in C, into the temporary root filesystem. The emphasis on static compilation stems from the desire for a self-contained image, eliminating dependencies on external shared libraries within the container environment. The specific commands used to copy the binary and required libraries, such as libc, are explicitly provided. The author underscores the importance of including only the strictly necessary files within the image to minimize its size and enhance efficiency.
After setting up the root filesystem, the post delves into the specifics of creating the container image itself. This involves employing the tar command to package the contents of the temporary directory into a tar archive. This archive, importantly, utilizes the -cf options to create an uncompressed archive, maintaining the original file structure and permissions. Subsequently, the generated tar archive is further processed using the gzip command to achieve compression, resulting in the final container image. The author stresses that this image is now ready to be run using low-level container runtimes like runc.
The post proceeds to explain how to execute this newly created container image. This involves utilizing runc and specifying configuration details in a config.json file. The configuration file defines essential parameters such as the container's process, memory limits, and root filesystem location. The structure and content of this configuration file are thoroughly described, emphasizing the importance of accurate configuration for successful container execution. The post concludes by demonstrating how to run the container using runc run, successfully executing the "hello world" program within the isolated container environment. The entire process highlights the intricate details of container image construction, providing a deeper understanding of the technology beyond the abstractions provided by higher-level tools like Docker. This hands-on approach clarifies the fundamental principles underlying containerization.
Summary of Comments ( 43 )
https://news.ycombinator.com/item?id=43396172
HN users largely praised the article for its clear and concise explanation of container image internals. Several commenters appreciated the author's approach of building up the image layer by layer, providing a deeper understanding than simply using Dockerfiles. Some pointed out the educational value in understanding these lower-level mechanics, even for those who typically rely on higher-level tools. A few users suggested alternative or supplementary resources, like the book "Container Security," and discussed the nuances of using
tarfor creating layers. One commenter noted the importance of security considerations when dealing with untrusted images, emphasizing the need for careful inspection and validation.The Hacker News post "Build a Container Image from Scratch" (https://news.ycombinator.com/item?id=43396172) has generated a moderate discussion with several insightful comments related to the process of building container images and some of the nuances involved.
One commenter points out the critical distinction between a container image and a running container, emphasizing that the article focuses on building the former. They highlight that a container image is essentially a template, while a running container is an instance of that template. This comment helps clarify a fundamental concept for those new to containerization.
Another commenter praises the article for its clear explanation of the image building process, specifically appreciating the step-by-step approach and the explanation of the importance of a statically linked binary for the entrypoint. They find it a valuable resource for understanding the underlying mechanisms involved.
Several commenters discuss the practicality and security implications of building container images from scratch. While acknowledging the educational value of understanding the fundamentals, they caution against using this approach for production environments. They argue that leveraging established base images and build tools offers better security, maintainability, and optimization. Building from scratch significantly increases the risk of inadvertently introducing vulnerabilities and requires considerable effort to replicate the functionality provided by standard base images.
The discussion also touches on the complexity of building images for different architectures, particularly when statically linking libraries. One comment emphasizes the challenges of cross-compiling and managing dependencies for multiple architectures, advocating for solutions like Docker's buildx for multi-arch builds to handle these complexities more efficiently.
Finally, some comments delve into more technical aspects of containerization, such as the role of the kernel and the use of tools like
stracefor debugging containerized applications. These comments provide deeper insights for those seeking a more advanced understanding of container internals.Overall, the comments section provides valuable perspectives on building container images from scratch, ranging from clarifying fundamental concepts to discussing practical considerations and more advanced technical details. While acknowledging the educational benefits, the general consensus leans towards utilizing established base images and tools for production deployments due to security and maintainability advantages.