How can Docker run distros with different kernels?

Question

How can docker run on a Debian host maybe an OpenSUSE in a container? It uses different kernel, with separated modules. Also older Debian versions have used older kernels, so how can run it on a kernel version 3.10+ ? Older kernels have only older built in functions, how can an old distro manage new features? What is "the trick" in it?

Answer 1

How can docker run on a Debian host maybe an OpenSUSE in a container

Because the kernel is the same and will support the Docker engine to run all those container images: the host kernel should be 3.10 or more, but its list of system calls is fairly stable.

See "Architecting Containers: Why Understanding User Space vs. Kernel Space Matters":

1. Applications contain business logic, but rely on system calls.
2. Once an application is compiled, the set of system calls that an application uses (i.e. relies upon) is embedded in the binary (in higher level languages, this is the interpreter or JVM).
3. Containers don’t abstract the need for the user space and kernel space to share a common set of system calls.
4. In a containerized world, this user space is bundled up and shipped around to different hosts, ranging from laptops to production servers.
5. Over the coming years, this will create challenges.

From time to time new system calls are added, and old system calls are deprecated; this should be considered when thinking about the lifecycle of your container infrastructure and the applications that will run within it.

See also "Why kernel version doesn't match Ubuntu version in a Docker container?":

There's no kernel inside a container. Even if you install a kernel, it won't be loaded when the container starts. The very purpose of a container is to isolate processes without the need to run a new kernel.