ci: build multi arch images with buildx

[rr-nick-tan]

What this PR does / why we need it:

build multi arch images with buildx fixes #335

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes:

Checklist chart

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [ ] Chart Version bumped
• [ ] Variables are documented in the README.md

[rr-nick-tan]

@jtblin , pls review

when you create the git tag, it triggers the image build and publish to Docerhub it works in my fork https://github.com/rr-nick-tan/kube2iam/runs/5441826765?check_suite_focus=true to make the release work, you need to provide the github secrets DOCKERHUB_USERNAME and DOCKERHUB_TOKEN