Enable ARM32v7 Container Image Builds

[seanmalloy]

Add ARM32v7 in addition to the currently supported architectures. This
will allow running descheduler on Raspberry Pi v3 devices.

Fixes #475

[seanmalloy]

/kind feature
/hold

This should work, but it doesn't. The docker manifest cannot be pushed. I think it's close, but not quite close enough.

[seanmalloy]

Looks like the docker version on my work laptop is too old to get the ARMv7 docker manifest to work.

Using a newer version of docker works correctly:

[damemi]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: damemi, seanmalloy

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[seanmalloy]

Just need to test that I can pull the images from my test registry and update the end user docs.

[seanmalloy]

I confirmed that the AMD64 image works. I have not way to test the ARM64 or ARMv7 images. I'll add the end user docs for the ARMv7 image to the end user guide closer to the v0.21.0 release.

/hold cancel

[seanmalloy]

/cc @ingvagabund @lixiang233

[seanmalloy]

FYI @cerebrate this enables the ARM32v7 images that you requested.

[lixiang233]

/lgtm