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this post was submitted on 15 Jul 2023
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Docker is not platform independent. The OS is not included in the image, and the executables in the image rely on the host system having a compatible kernel and system architecture. Only userspace components like software libraries and, unfortunately, CA certificates are included in the image.
It primarily supports x86_64 Linux systems. If you want to run on ARM, you need special images or you need a CPU emulator. If you want to run on Mac OS or Windows, you usually need a VM. There are Windows Docker containers, but Docker as a technology isn't really applicable to Windows because of the dirty separation between userspace and the kernel (if you've ever tried to run Docker on Windows Server without Hyper-V support, this is why it's so difficult to get it to work and it stops working after Windows updates).
Let me introduce you to
Docker buildx, you don’t know her, she’s from Canada. But seriously, multi-platform images are a thing, and I need to figure out how to do themYou can create multi-platform images (actually manifests of single-platform images) without buildx, and buildx isn't enough to create multiplatform images. In its default configuration, buildx can usually build images for different processor architectures but requires CPU emulation to do it. If the Dockerfile compiles code, it runs a compiler under emulation instead of cross compiling. To do it without CPU emulation involves configuring builders running on the other platforms and connecting to them over the network.
I don't know if it supports building images for multiple operating systems, but it probably doesn't matter. I've only ever seen container images for Linux and Windows, and it's virtually impossible to write a single Dockerfile that works for both of those and produces a useful image. The multiplatform images that support Linux and Windows are probably always created using the manifest manipulation commands instead of buildx.
This is true. However many big maintained public images are multi-arch so down for ARM, and the fact that Docker runs in a VM on Windows and OSX when you install it doesn’t matter to most people. On Linux indeed it reuses the host’s kernel (which is why containers can be a lot lighter than VMs)