Does Docker image include OS?

user_574122 0
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I have below Dockerfile"

FROM openjdk:12.0.2

EXPOSE 8080

ADD ./build/libs/*.jar app.jar

ENTRYPOINT ["java","-jar","/app.jar"]

The resulting Docker image encapsulates Java program. When I deploy this Docker image to Windows Server or Linux, does the image always include OS like Linux which runs on top of host OS (Windows Server or Linux) ?

I am asking this question in the sense of Docker image being physical box which contains other boxes (one being openjdk), does this box also contain Linux OS box that I can pull out of it ( assuming if this was possible) and install it as Linux OS on empty machine?

docker
3 Answers

That depends on what you call the "OS". It will always contain stuff from the distribution image, it is built on.
For example, a debian based images will include apt and other debian-specific tools. But most of the stuff you need on a "complete" machine (as in non-container), will have been removed to keep the image as small as possible.

It will not contain the kernel, as it is running on the host machine and is controlled by the host's kernel.

user_36836390

It doesn't include the entire operating system, but the image will be dependent on either linux or windows, you can't build an image that runs on both in one Dockerfile. The reason for the dependency is that a docker container shares resources with it's host machine in a carefully fenced off way, this mechanism is different on windows and linux (though to you, as a docker user, the difference is invisible).

user_589910

The "official" OpenJDK images from the Docker Hub are available in variants based on a number of different Linux distributions. There is a cut-down Debian, an Alpine, and others. There are advantages and disadvantages to each.

The image will need to contain enough operating system dependencies to allow the JVM to run. It may also include basic diagnostic and management tools -- enough to carry out rudimentary troubleshooting in the container, anyway. You can expect all the images to contain at least basic console shell tools like "cp" and "cat", although they differ in implementation. For example, the Alpine variant gets these utilities from BusyBox, not from a conventional GNU/Linux installation.

It's possible to create a Docker image that contains no platform dependencies at all, but there's little incentive to be that minimal -- you'd just have to build more stuff into the application program itself.

user_84712080
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