Flatcar Container Linux is a fully open source, minimal-footprint, secure by default and always up-to-date Linux distribution for running containers at scale. It is shipped as an image and does not include any package management. OS binaries and libraries reside on a read-only partition. This talk will briefly introduce Flatcar’s core concepts before diving into a detailed discussion on modifying, testing, and vending your own OS images using the SDK. This talk will discuss building, modifying, and testing Flatcar Container Linux OS images. It will introduce the innerworks of the OS image building process and will enable the audience to craft their own images. While running (release-)tests will also be briefly covered the focus is on modifying and building. Target audience are engineers and tinkerers interested in OS building and in vending immutable OS images that fit their own needs. Flatcar Container Linux is a minimal Linux distribution optimised for running containers at scale. It ships OS images – both for installation and update – and lacks a package manager. Binaries in the OS partition (at “/usr”) are immutable after installation. Therefore, modifying the OS to suit one’s needs can be challenging, and deeper changes to the OS will require vending one’s own images. After a brief intro to Flatcar’s Gentoo roots, build process, and SDK, this talk will dive into understanding sources and repository structure, running builds, smoke-testing custom images, and running full-blown release tests. We will include a number of pre-recorded CLI demos in the talk to aid understanding. The talk will close with an outlook of how to bring your custom images to production.