Do all docker images have minimal OS?

Question

I am trying to learn Docker and for that referring to online materials. I came to know that there is official hub of images which we can pull, and run a container.

The repos are available at https://hub.docker.com/ , part of screen shot:

In this diagram we can see the official images of ubuntu, httpd, mysql (and so on).

My question is:

Do all these images have "minimal OS" on which they run. For example, if we consider httpd image, does it have the needed OS on which it runs?

Answer 1

From my understanding images are built in a layered architecture from a parent image. So we have a parent image and then the changes for this image is one more layer above parent image. If you see dockerfile for an image you can see something like this

FROM node:6.11.5

This node:6.11.5 is a parent image for our current image.

If you check dockerfile of parent images you will find they are somewhere in the hierarchy follow from base image.

This base image is basically an OS without kernel but has only userland software based on the different linux distributions(eg, centos, debian). So all the images uses the host OS kernel. Hence, you cannot install a Windows container on a Linux host or vice-versa.

So basically all images are layered changes on the base image which is an OS without kernel.

Please find below links for further information:

https://serverfault.com/questions/755607/why-do-we-use-a-os-base-image-with-docker-if-containers-have-no-guest-os

https://blog.risingstack.com/operating-system-containers-vs-application-containers/

If you need to create a base image you can see the steps here.

https://docs.docker.com/develop/develop-images/baseimages/

Please correct me if i am wrong.

Answer 2

Most images are based on a distribution as you can see it in their Dockerfiles. Except for the distribution images themselves. They have a different base-image, which is called scratch.

You can review the images they are based on when you visit the project's page on DockerHub, for example https://hub.docker.com/_/httpd/

Their Dockerfiles are referenced and you can review them by clicking on them, e.g. the first tag "2.2" refers to this file. The first line in the Dockerfile is FROM debian:jessie and shows, that it is based on a Debian image.

It is widely used to have a separated tag with the postfix -alpine in it to indicate, that alpine linux is used, which is a much smaller base-image than the Debian image. This leads to a smaller image of the httpd image, because the base-image is much smaller.