Docker container isolation, does it care about underlying Linux OS?
If I run Docker Engine and the same container on a set of different Linux distributions, will the container run in the same way? I am asking because in many cases applications depend on a specific Linux distribution for some resources, such as fonts. If my application running inside a Docker container depends on a font used in Ubuntu (and there may be many other dependencies), how is this managed? Will I need to install the font inside container, will I need to run Ubuntu inside the container running the application, or does the application use fonts from the underlying OS running the container?
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Any missing resources should be installed in a Docker image (which can start from the ubuntu image).
It should not rely on host for dependencies.
The idea is to be able to reproduce the environment each time a container is run from an image.
A container don’t see the host resources (beside mounted volumes), since it has the Docker engine between the container and the host, in order to configure cgroups and namespaces to control which resources the container can see and access.
The “fedora” image referenced in jboss/base is the base image:
In Docker terminology, a read-only Layer is called an image. An image never changes.
Since Docker uses a Union File System, the processes think the whole file system is mounted read-write. But all the changes go to the top-most writeable layer, and underneath, the original file in the read-only image is unchanged.
Since images don’t change, images do not have state.
See “What is the relationship between the docker host OS and the container base image OS?”:
The only relationship between the host OS and the container is the Kernel.
as the kernel is still the kernel of the host, you will not have any specific kernel module/patches provided by the distribution.
What you need to be careful is
- the kernel dependency,
- and some mandatory access control (SELinux, Apparmor) configurations, which are distribution dependent and may have an impact on how your Docker containers work.