Docker: Up & Running, 3rd Edition

Chapter 4. Working with Docker Images

Every Linux container is based on an image. Images are the underlying definition of what gets reconstituted into a running container, much like a virtual disk becomes a VM when you start it up. Docker or Open Container Initiative (OCI) images provide the basis for everything that you will ever deploy and run with Docker. To launch a container, you must either download a public image or create your own. You can think of the image as a single asset that primarily represents the filesystem for the container. However, in reality, every image consists of one or more linked filesystem layers that generally have a direct one-to-one mapping to each build step used to create that image.

Because images are built up from individual layers, they put special demands on the Linux kernel, which must provide the drivers that Docker needs to run the storage backend. For image management, Docker relies heavily on this storage backend, which communicates with the underlying Linux filesystem to build and manage the multiple layers that combine into a single usable image. The primary storage backends that are supported include the following:

• Overlay2¹

• B-Tree File System (Btrfs)

• Device Mapper

Each storage backend provides a fast copy-on-write (CoW) system for image management. We discuss the specifics of various backends in Chapter 11. For now, we’ll use the default backend and explore how images work, since they make up the basis for almost everything ...