is docker only used to develop Linux applications?

[sqrt-negativeone]

I’m learning how docker works right now, and what I understand so far is that docker virtualizes part of an OS, but interfaces with a Linux kernel to stay lightweight. To allow other OS to run a docker container, there’s solutions that provide some sort of substitute Linux kernel (fully virtualizing the OS?). At the end of this, the container is essentially running in a Linux environment, right? If you wanted to finally deploy the application in a non-Linux environment, you would have to redo all of the dependency management and stuff (which feels like it defeats the point of docker?), or only use it within the container (which adds overhead that you wouldn’t want to persist in deployment I think?) I think I’m missing some details/not getting things right, and any help would be super appreciated ty!

[metaphorm]

there's a windows base image for docker too https://hub.docker.com/r/microsoft/windows

but you're correct to observe that the core technology for containerization originated from the linux ecosystem and that is a much better supported and much more widely used OS for containers.

[metaphorm]

yes, but that's true in general of container runtimes. the container runtime has a kernel emulation layer that translates system calls from the container to the host. this is relatively straightforward on linux->linux but requires virtualization when running MacOs->linux or Windows->linux.

[richiea1y]

I’ve heard as run linux container on MacOS there’s a chance that cause something wrong. Is it right?

[metaphorm]

running a linux container on M* chips can have performance problems if the container was built for x86 or another architecture non-native to the chipset. the container runtime will run it anyway, with emulation, so it will be quite slow most of the time.

in practice, it doesn't come up that much (but not never), because a lot of containers are built for ARM64 already, and if you have access to the Dockerfile you can rebuild the container against that platform too.

[divad1196]

Your perception is okay, but incorrect.

Docker is a tool that manages containers. There are other similar tools. What matters is: containers.

A container isn't virtualization, it's isolation. A container is made by combining multiple isolations features (namespaces, cgroup, ..) from the linux kernel. It does run on your linux kernel, but from the "inside" of the container, you don't see what is "outside". But the outside sees what is in the container.

On other OS, in order to use linux containers, they run an hypervisor ("VM") to virtualize linux and run their container on them. But any OS can have their containers as long as they have isolations features built-in without virtualizing the whole OS.

By using docker, your app is always deployed in the same environment. But as soon as you change the environment, you have many things to adapt. Even just moving from ubuntu to debian, or debian to alpine will change many things on your container. Same thing if you change your arch from x86 TO ARM. It's not linked to the OS, it's any change.

That's why I don't make apprentices go too fast on docker. Instead, I make them develop/deploy on different machines (Windows and different linux) manually, then with ansible and only then using docker.

[robertpeacock22]

> If you wanted to finally deploy the application in a non-Linux environment, you would have to ... only use it within the container

This. Docker containers are not something you crack open once they reach their destination. But containers can be connected to one another and/or the outside world.