Are containers specific to a host OS?

Question

Are containers specific to a particular host OS? For instance, if a container is created on Windows with particular dependencies (e.g., DLL files), can it run in a setup in which the host OS is Linux? I initially assumed that a container must be specific to a particular host OS.

But the following two excerpts seem to suggest that I may not have understood the mechanics correctly. So my question is: are containers built over the docker engine so when the dependencies are included, they are relative to the docker engine and the underlying host OS does not matter?

(1) From IBM:

Containerization allows developers to create and deploy applications faster and more securely. With traditional methods, code is developed in a specific computing environment which, when transferred to a new location, often results in bugs and errors. For example, when a developer transfers code from a desktop computer to a virtual machine (VM) or from a Linux to a Windows operating system. Containerization eliminates this problem by bundling the application code together with the related configuration files, libraries, and dependencies required for it to run. This single package of software or “container” is abstracted away from the host operating system, and hence, it stands alone and becomes portable—able to run across any platform or cloud, free of issues. [https://www.ibm.com/cloud/learn/containerization]

(2) From Docker:

Does Docker run on Linux, macOS, and Windows? You can run both Linux and Windows programs and executables in Docker containers. The Docker platform runs natively on Linux (on x86-64, ARM and many other CPU architectures) and on Windows (x86-64).

Docker Inc. builds products that let you build and run containers on Linux, Windows and macOS.

What does Docker technology add to just plain LXC?🔗 Docker technology is not a replacement for LXC. “LXC” refers to capabilities of the Linux kernel (specifically namespaces and control groups) which allow sandboxing processes from one another, and controlling their resource allocations. On top of this low-level foundation of kernel features, Docker offers a high-level tool with several powerful functionalities:

Portable deployment across machines. Docker defines a format for bundling an application and all its dependencies into a single object called a container. This container can be transferred to any Docker-enabled machine. The container can be executed there with the guarantee that the execution environment exposed to the application is the same in development, testing, and production. LXC implements process sandboxing, which is an important pre-requisite for portable deployment, but is not sufficient for portable deployment. If you sent me a copy of your application installed in a custom LXC configuration, it would almost certainly not run on my machine the way it does on yours. The app you sent me is tied to your machine’s specific configuration: networking, storage, logging, etc. Docker defines an abstraction for these machine-specific settings. The exact same Docker container can run - unchanged - on many different machines, with many different configurations.

Answer 1

The host OS, or precisely, the kernel provided still matters. That's why you can't run Windows containers on Linux. You can run Linux container on Windows due to Hyper-V and WSL2, and on macOS with Hypervisor, but that's it. If the provided kernel is compatible (doesn't have to be identical), usually similar version and the same architecture (remember, there are x64, ARM64, etc) or at least supported virtualization (x64 containers can run on M1, which is ARM64) then you can just run the container, no need to worry about DLLs because they're supposed to be included either in one of the base image you start with or the image you generate.