Multi-platform build targeting both amd64 and arm64

[odsod]

What are you trying to do?

Trying to set up a build to output multi-platform images for both linux/arm32v5 and linux/amd64 for the same main entrypoint.

I can configure one or the other by changing the default base image between gcr.io/distroless/static and gcr.io/distroless/static:latest-arm64, but that requires changing the base image configuration between the build steps.

What did you expect?

Lacking a distroless/static image that supports all platforms, the ability to configure different base images for different platforms in the same build.

[imjasonh]

Yeah, this is a known annoyance. Some discussion and prototyping in #475

Tekton also does this, to combine distroless and a Windows base image as a pre-step, but it's definitely annoying.

I think we could figure out how to make it work in ko directly.

[imjasonh]

Actually: Does this work for you?

ko publish ./cmd/app --platform=linux/amd64,linux/arm

If distroless/static supports armv5 I'd expect this to work.

[odsod]

@imjasonh Thank you, that seems to work!

So if I read this PR correctly I'm supposed to provide the ARM instruction set as part of the platform, #239 - but that leads to a base image that is not included in the distroless/static metadata?

So to solve this I can configure the build to target linux/arm and manually set GOARM=5?

[imjasonh]

You can specify linux/arm/v5 to differentiate between variants if the base image specifies multiple (linux/arm/v5, v7, whatever). That PR means that the base image's detected/supported variant would be used to further specify better go compilation support if it can find it (using GOARM).

Since distroless only currently provides a generic linux/arm, this is moot. If you want to use another base image, eg alpine, you could.

If you want to support linux/arm via distroless/static and other variants via eg alpine, you could combine a base image together, like Tekton does as described in the issue above, but that's cumbersome. ko could do better.

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