Without this change, when building images with --cache-to-registry,
BuildKit uses a proprietary cache artifact format, which breaks
when using third-party registries such as Harbor or ECR.
By adding the image-manifest=true option, BuildKit uses an
OCI-compliant cache artifact format that should be compatible with all
registries. This option requires BuildKit 0.12 or later (check with
"docker buildx ls").
See https://github.com/goharbor/harbor/issues/18941 and
https://github.com/moby/buildkit/issues/2251 for background
information.
Co-authored-by: Andrés González <andres@aulasneo.com>
Fixes #1118.
