does docker always need an operating system as base image

Question

I have heard that docker doesn't need a separate os in linux, because it shares with the host os, but in hyper-v Windows it can run Windows OS because it can hyper a linux virtual machine so run linux software on it.

But, I get confused about the FROM stage in the dockerfile, all guides said like this:

FROM ubuntu:18.04
cp . /usr/local/bin
RUN make
CMD /usr/local/bin/youapp

I can understand this step, first you need an OS, then you deploy your application; finally you run your app or whatever.

But what does the FROM stage really mean?

Does it always need an OS? Does nginx docker image have an os in it?

If i want to build my own app, I write it, I compile it, I run it; but does my own app need an OS? If not, what should I write in the FROM stage?

i got this picture, it said docker container does not need os,but use the host os,now docker build always need an os

Answer 1

The containers on a host share the (host's) kernel but each container must provide (the subset of) the OS that it needs.

In Windows, there's a 1:1 mapping of kernel:OS but, with Linux, the kernel is bundled into various OSs: Debian, Ubuntu, Alpine, SuSE, CoreOS etc.

The FROM statement often references an operating system but it need not and it is often not necessary (nor a good idea) to bundle an operating system in a container. The container should only include what it needs.

The NGINX image uses Debian (Dockerfile).

In some cases, the container process has no dependencies beyond the kernel. In these cases, a special FROM: scratch may be used that adds nothing else. It's an empty image (link).

Answer 2

No its not like that. To create any docker image using DockerFile, You need to start with a base docker image. That base docker image can be anything, Like an empty image as well, In the docker file in your example the FROM section says ubuntu, it means its assuming ubuntu as the base image. Its not always needed to have an OS as base image.

Follow this link - https://linuxhint.com/create_docker_image_from_scratch/ This will clear your doubts related to base image.

Answer 3

now i got answer

the From stage import the software but not the OS with kernel

it just provide a platform for your application,the ubuntu,debian,centos you write in FROM stage is just a software,the true kernel does not have relationship with them.

so if your application can run dependent ,it must like hello-world ,just a binary-package,dont rely on any other library. but mostly you need an OS,because they have the library you need.