Switches to using buildx properly for multi-arch

[Stringy]

Description

In theory this is a simpler mechanism for multi-arch but may be slower since each arch is built in sequence. To counteract this a little, each container is now built in parallel

Checklist

• Investigated and inspected CI test results
• Updated documentation accordingly

Automated testing

• Added unit tests
• Added integration tests
• Added regression tests

If any of these don't apply, please comment below.

Testing Performed

TODO(replace-me)
Use this space to explain how you tested your PR, or, if you didn't test it, why you did not do so. (Valid reasons include "CI is sufficient" or "No testable changes")
In addition to reviewing your code, reviewers must also review your testing instructions, and make sure they are sufficient.

For more details, ref the Confluence page about this section.

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[github-actions]

Kernel	Method	Without Collector Time (secs)	With Collector Time (secs)	Baseline median (secs)	Collector median (secs)	PValue
rhel.rhel-8	ebpf	215.208	232.503	204.92	213.7	🟢
ubuntu-os.ubuntu-2004-lts	ebpf	227.655	249.467	236.1	234.97	🟢
ubuntu-os.ubuntu-2204-lts	ebpf	232.863	262.903	222.56	229.05	🟢

[ovalenti]

LGTM !

There is just the question of the grpc-mock image bump. If this let's your s390x attempt pass, that would be a good reason to keep it 😉