What is the Docker ancestor image?
I am in the middle of Advent of Code 2018, a great daily programming challenge. I will also be working on the site and on more coding practice, so I may reduce my activity on the blog in the near future.
TL;DR The ancestor image is
FROM scratch, a no-op command.
I have had many opportunities to introduce Docker to my coworkers in depth, although I had never received any sort of training. Rather I relied on Internet resources.
And so I have had some difficulty answering one question that each of my coworkers would inevitably ask as I explain the layout of a Docker image.
The way I do it is simply going over a Dockerfile, such as this one which is basically the
Dockerfile for Telegraf, the metrics agent:
As I introduce the
FROM operator, I mention the notion of base image, which is explained here. Then, the question occurs: “So what is the ancestor image? What happens if I check the parent image of the parent of the parent…” It obviously comes to a stop eventually.
Let’s take the
Dockerfile above as an example: it is built on top of Alpine Linux, a minimal Linux distro that is very popular for building containers that use little disk space and RAM. It ships with basic utilities like BusyBox for a mere 5MB volume.
So how is Alpine Linux built? It’s a bit complicated because of the automated versioning and build process, but it comes down to this: a root filesystem named
rootfs.tar.xz is put together and then the following Dockerfile builds the Alpine Linux image:
The key is the first line:
FROM scratch. As of docker 1.5,
FROM scratch is a no-op command. It is designed for images that do not need an actual parent image. That could be either a user space like Debian or Alpine Linux that ships with interpreters like
bash and other utilities usually taken for granted, or just straight-up binaries to make the most minimal images possible.