Docker: why installing a linux dist inside the container?

I am starting to understand docker and as far as I am aware the docker container runs on the defualt linux dist where the container is installed – in my case its a mac os x lightweight dist thats comes with docker toolbox.

So why do i see many docker files actually installing a distrib inside the container, does this not defeat the object of keeping things light?

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  • For example, here is one docker file starting with …

        FROM debian:jessie

    so this is installing a docker image inside the container which is based off of debian.

    I also see many others usign ubuntu, for example

    Can this step not be bypassed and software installed directly in the container use the underlining linux dist where the container is installer?

    Thanks in advance for any clarification

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  • One Solution collect form web for “Docker: why installing a linux dist inside the container?”

    Because, just as for physical or virtual machines, setting up a userland environment is going to be a pain without a distribution.

    This is, IMO, one of the strong benefits of docker: Pick the most suitable distribution for a particular application.

    A containerized application is probably going to have dependencies. To install these dependencies, it helps a lot to have a package manager. Some dependencies are also included by default in many distributions, which makes it a good idea for the container creator (application) to choose its own distribution.

    Additionally, remember that packaging a whole distribution does not necessarily waste a lot of resources:

    • Docker images are stored as deltas against a common baseline, meaning that two images based on debian:jessie could reuse the same data for the baseline.
    • Distributions are actually not that large, as they are usually minified versions of the full system images.

    If you really want to create a minimal image, try busybox. However, you will oftentimes find yourself outgrowing it quite fast for any real world container image.

    Docker will be the best open platform for developers and sysadmins to build, ship, and run distributed applications.