Arm64 support

[mflis]

Changes that allow building Rosco images for both amd64 and arm64 architectures. That's the first of many PRs to various Spinnaker Components that allow building docker images for multiple architectures.

Motivation:
Allow running & experimenting with Spinnaker locally on Macbok M1.

Changes:

• Choose binaries in docker build based on the target architecture
• bump Kustomize to a version that contains arm64 binaries
• update Github action to build multiple architectures

[spinnakerbot]

The following commits need their title changed:

• 8e9530f: Enable multi architecture builds

• 51e7a27: Build docker images for multiple architectures

Please format your commit title into the form:

<type>(<scope>): <subject>, e.g. fix(kubernetes): address NPE in status check

This allows us to easily generate changelogs & determine semantic version numbers when cutting releases. You can read more about commit conventions here.

[dbyron-sf]

@mflis after merging this the build that runs on master failed: https://github.com/spinnaker/rosco/runs/6821708648?check_suite_focus=true . Mind taking a look?

[kskewes-sf]

This is great, paves the way for better local development on newer arch and also experimenting with other instance types available from cloud providers.