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Reputation: 2904

Docker base images, what do they compose of?

I am trying to wrap my head around the Docker architecture, in particular figuring out what exactly a base image consists of, and in doing I have been exploring some of the images found on the docker hub. Specifically when looking at the following repo it references the centos-7.2.1511-docker.tar.xz file. I've downloaded and examined the contents of the tar and it has your typical Linux filesystem.

As I understand it, this is not a complete Linux OS and is just a replica of a linux filesystem with all the non essentials removed? Where all other requirements are drawn from the Host OS when a container is run(?)

My question essentially boils down to how one would go about creating that tar file? What exactly do you need. My intention is not to create one but rather understand what portion of files/data/dependencies come from a target OS to create an image and what gets used on the Host OS

Upvotes: 2

Views: 188

Answers (1)

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137398

A Docker container is a set of processes, running a sandbox enabled by Linux namespaces, on top of the host kernel.

A Docker image is a set of layers, which are often simply tarballs, of files that are unpacked, and made to look as if they are the root of the filesystem when used to start a container.

A Docker image could be just a single statically-linked executable! You can create your own Docker image from scratch by simply creating a tarball of a single executable, and giving it to docker load which wI'll store it as the appropriate internal format and register it as an image.

As you can see then, a Docker image need not be much. It certainly doesn't need a kernel, or any of the components normally used for configuring the system, networking daemons, or even things like cron. Those are all left to the host.

Things that are usually available in an image are a dynamic library runtime, and files like /etc/hosts, /etc/resolv.conf, and other files which are referenced directly by libc. This allows you to add typical dynamically-linked executables which interact with the system as if they're running on a traditionalal OS.

I have successfully "Dockerized" a legacy CentOS 6-based VM by uninstalling as many packages as possible, then tar-ing up the filesystem (excluding directories like /proc, /sys, /dev, etc.) and loading this via docker load. Afterwards, I started a container and (sometimes forcefully) removed additional "system" packages that serve no purpose in a Docker image, like kernel, udev, etc.

This blog post goes into some of the specifics of docker load:

Upvotes: 2

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