Build multi-arch docker images

[antonincms]

Fixes issue #939

Proposed Changes

On a M1 / M2 mac the performance are 5x worse for our use case if using the amd64 docker.
We would like to have the possibility of using a multi-plateform image, and this PR allows that :

• Update the Dockerfile to use the TARGETARCH environment variable to pull the correct version of go.
• Update the docker-image github action pipeline to setup QEMU and buildx,
  and then building multi-plateform images of kapitan.

Building an image on a computer of the same architecture should work as previously.
I could add some documentation on how to do locally what the CI does (build a multi-arch image with a single tag).

This was inspired by this bit of documentation about building multi-plateform images : https://docs.docker.com/build/ci/github-actions/examples/#multi-platform-images
There is also this more general documentation about how to set this up outside of CI : https://docs.docker.com/build/building/multi-platform/

[uberspot]

Overall looks good. I've rebased to re-run tests.
Also I could add some documentation on how to do locally what the CI does (build a multi-arch image with a single tag).
It would be indeed beneficial if you added a pointer for this. Maybe as a comment in the makefile or in contributing.md. Either works (?)

[antonincms]

I added some comments in the CI and a paragraph in the README, is it ok ?

[ramaro]

looks good, thank you @antonincms !

[antonincms]

Thank you for the review !