“Building Docker Image From Scratch” is easy way to build any kind of containerization image. Let’s get bit more deep into it and understand what it is doing at the low level. Everyone knows in Linux everything is considered as file only. The whole operating system is nothing but a collection of files and folders.

In the previous post, we have seen there are 2 steps required as per “Dockerfile” and same has been embedded in the container layer file system. As a result we have got a container image which is nothing but a collection of layered filesystem.

Container images are templates from which containers are created. These images are not just one monolithic block, but are composed of many layers. The first layer in the image is also called the base layer:

Each layer is mapping with one command and this command is nothing but a file which will be stacked in this image. The all layers of container images are immutable or read only which means once created can’t be changed but we can delete it. In case we want to use the content of one layer in another layer, in that case we have to copy it from layer and use it in new layer. 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/.”

When the docker engine creates a container from these images, it adds writable layer on the top of immutable or read only layers like as shown in below image: