fix cross compilation for arm platforms

[crazy-max]

follow-up docker/docker-ce-packaging#665 (comment)

cross compilation is currently broken against some arm platforms:

#43 0.132 
#43 0.138 ---> Making bundle: cross (in /build/bundles/cross)
#43 0.139 Cross building: /build/bundles/cross/linux/arm/v6
#43 0.182 Building: /build/bundles/cross/linux/arm/v6-daemon/dockerd-0.0.0-20220329083112-174e51c
#43 0.182 GOOS="linux" GOARCH="arm" GOARM="6"
#43 76.80 # github.com/docker/docker/cmd/dockerd
#43 76.80 loadinternal: cannot find runtime/cgo
#43 76.80 /usr/local/go/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
#43 76.80 gcc: error: unrecognized command-line option '-marm'; did you mean '-mabm'?

- What I did

fix cross compilation for arm platforms by setting correct switches for CGO_CFLAGS and CGO_CXXFLAGS flags.

also adds a GitHub Action job to check cross targets work, which is not currently done in Jenkins.

- How to verify it

$ mkdir -p autogen # workaround that will be fixed by https://github.com/moby/moby/pull/43431
$ DOCKER_CROSSPLATFORMS="linux/arm/v7 linux/arm/v6 linux/arm/v5" docker buildx bake cross

- Description for the changelog

fix cross compilation for arm platforms

---

in a follow-up we should switch to xx.

cc @thaJeztah

[thaJeztah]

LGTM

Are any of these perhaps also needed to address docker-library/docker#260 ?

[thaJeztah]

Should we remove the cross stage from Jenkins once this is merged?

remove --build-arg CROSS=true from

moby/Jenkinsfile

Line 90 in c687298

sh 'docker build --force-rm --build-arg APT_MIRROR --build-arg CROSS=true -t docker:${GIT_COMMIT} .'

remove the cross stage:

moby/Jenkinsfile

Lines 167 to 179 in c687298

	stage("Cross") {
	steps {
	sh '''
	docker run --rm -t --privileged \
	-v "$WORKSPACE/bundles:/go/src/github.com/docker/docker/bundles" \
	--name docker-pr$BUILD_NUMBER \
	-e DOCKER_GITCOMMIT=${GIT_COMMIT} \
	-e DOCKER_GRAPHDRIVER \
	docker:${GIT_COMMIT} \
	hack/make.sh cross
	'''
	}
	}

[crazy-max]

I think we should remove the CROSS and DOCKER_CROSSPLATFORMS args in a follow-up and rely only on --platform like docker/cli, buildx so packaging will be easier and also we could remove the cross logic in the .binary script. But that needs some work to align the dependencies in the dockerfile.

[thaJeztah]

We need to make sure it can be run outside of the Dockerfile / container as well; it's possible there's people using the scripts to build/cross compile directly on the host.

(I guess xx can be used in that situation as well, but may need either extra instructions or a script to set it up)

[tonistiigi]

We do not need a non-Dockerfile "cross-all" type target if we have no use for it ourselves. The script for the single arch would run for any target arch inside Dockerfile(and cross-all is just --platform list). To run the same script locally it is user's responsibility that they have a cross toolchain for the target arch they are setting.

[samuelkarp]

it's possible there's people using the scripts to build/cross compile directly on the host.

I don't know of distros that would use these scripts for cross-compilation, but I do know at least some distros definitely build their packages outside of containers/in their own environments.

[thaJeztah]

Let's continue that discussion when that change is actually made (this PR does not yet make that change)

[thaJeztah]

@tonistiigi @tianon PTAL

[thaJeztah]

@tonistiigi @tianon PTAL; this LGTY?

[crazy-max]

@thaJeztah

Are any of these perhaps also needed to address docker-library/docker#260 ?

Don't think it's an issue in this repo but in https://github.com/docker-library/docker/blob/master/Dockerfile.template.

My bad I see this Dockerfile downloads our static packages. Are these packages cross compiled or native? If cross then yes that should fix this issue.

[thaJeztah]

Thx! Let's get this one in