Publish linux/amd64 and arm64 images to GitHub Container Registry

[stefanprodan]

To avoid Docker Hub pull limits we should move to GitHub Container Registry and publish AMD64 and ARM64 images for the toolkit controllers:

• source-controller
• kustomize-controller
• helm-controller
• notification-controller

Reference implementation:

• Dockerfile: https://github.com/fluxcd/notification-controller/blob/master/Dockerfile
• GitHub workflow based on docker/build-push-action v2: https://github.com/fluxcd/notification-controller/blob/master/.github/workflows/release.yml

The controller images will be made public under ghcr.io/fluxcd/:

• linux/amd64: ghcr.io/fluxcd/<controller>:semver
• linux/arm64: ghcr.io/fluxcd/<controller>-arm64:semver

Once the controller images are available on GitHub Container Registry, we should add a command flag to gotk install/bootstrap that allows users to chose between amd64 and arm64 e.g.:

gotk install --arch=arm64