How Docker selects an image's os/arch

Solution 1:

I'm wondering what image I'll get if I just do a "docker pull python"

From "Leverage multi-CPU architecture support"

Most of the Docker Official Images on Docker Hub provide a variety of architectures.
For example, the busybox image supports amd64, arm32v5, arm32v6, arm32v7, arm64v8, i386, ppc64le, and s390x.
When running this image on an x86_64 / amd64 machine, the amd64 variant is pulled and run.

So it depends on the OS used when you do your docker pull.

and how I can choose the OS and architecture myself.

The same page adds:

You can also run images targeted for a different architecture on Docker Desktop.

You can run the images using the SHA tag, and verify the architecture.
For example, when you run the following on a macOS:

docker run --rm docker.io/username/demo:latest@sha256:2b77acdfea5dc5baa489ffab2a0b4a387666d1d526490e31845eb64e3e73ed20 uname -m

aarch64

docker run --rm docker.io/username/demo:latest@sha256:723c22f366ae44e419d12706453a544ae92711ae52f510e226f6467d8228d191 uname -m

armv71

In the above example, uname -m returns aarch64 and armv7l as expected, even when running the commands on a native macOS or Windows developer machine

Solution 2:

It's based on the host platform where docker is running. You can see this in docker info:

$ docker info --format '{{ .OSType }}/{{ .Architecture }}'
linux/x86_64

The code for this is in containerd and the OS and Arch are looked up slightly differently in go:

$ go env
GO111MODULE=""
GOARCH="amd64"
...
GOOS="linux"
...