Learn Docker：Fundamentals of Docker 19.x

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The layered filesystem

Container images are templates from which containers are created. These images are not made up of just one monolithic block but are composed of many layers. The first layer in the image is also called the base layer. We can see this in the following graphic:

The image as a stack of layers

Each individual layer contains files and folders. Each layer only contains the changes to the filesystem with respect to the underlying layers. Docker uses a Union filesystem—as discussed in Chapter 3, Mastering Containers — to create a virtual filesystem out of the set of layers. A storage driver handles the details regarding the way these layers interact with each other. Different storage drivers are available that have advantages and disadvantages in different situations.

The layers of a container image are all immutable. Immutable means that once generated, the layer cannot ever be changed. The only possible operation affecting the layer is its physical deletion. This immutability of layers is important because it opens up a tremendous amount of opportunities, as we will see.

In the following screenshot, we can see what a custom image for a web application, using Nginx as a web server, could look like:

A sample custom image based on Alpine and Nginx

Our base layer here consists of the Alpine Linux distribution. Then, on top of that, we have an Add Nginx layer where Nginx is added on top of Alpine. Finally, the third layer contains all the files that make up the web application, such as HTML, CSS, and JavaScript files.

As has been said previously, each image starts with a base image. Typically, this base image is one of the official images found on Docker Hub, such as a Linux distro, Alpine, Ubuntu, or CentOS. However, it is also possible to create an image from scratch.

Docker Hub is a public registry for container images. It is a central hub ideally suited for sharing public container images.

Each layer only contains the delta of changes in regard to the previous set of layers. The content of each layer is mapped to a special folder on the host system, which is usually a subfolder of /var/lib/docker/.

Since layers are immutable, they can be cached without ever becoming stale. This is a big advantage, as we will see.