Configure QEMU userspace emulation for Linux build

[sarandia]

The Dockerfile uses commands in Linux, which require the correct binfmt
for the architecture we build for. Therefore we need to configure Docker
to use QEMU userspace emulation to build images whose arches are
different from the host machine.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
This is a follow-up to #741. We need to configure Docker buildx correctly for the changes in #741 to work.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[k8s-ci-robot]

Hi @sarandia. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[sarandia]

@jingxu97 @mattcary PTAL, thanks!

[mattcary]

/assign @jingxu97

[mattcary]

/ok-to-test
/release-note-none

[sarandia]

/retest

[cpanato]

/lgtm
/hold for maintainers review

[msau42]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: msau42, sarandia

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[msau42]

/hold cancel

[jingxu97]

/lgtm

[sarandia]

/retest

[jingxu97]

/retest

[cpanato]

/retest