Add --metadata-file

Add a MetadataFile field to BuildOptions, to which we write a dictionary
of information about a just-committed image.

Pay more attention to sourceDateEpoch than to timestamp when we're
tagging an existing image with the intended destination name.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

internal/metadata/metadata.go

@@ -0,0 +1,22 @@
+package metadata
+
+import (
+	"github.com/containers/buildah/docker"
+	"github.com/opencontainers/go-digest"
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+// Build constructs a map containing the passed-in information about a just-committed or reused-as-cache image.
+func Build(imageConfigDigest digest.Digest, descriptor v1.Descriptor) (map[string]any, error) {
+	metadata := make(map[string]any)
+	if imageConfigDigest.Validate() == nil {
+		metadata[docker.ExporterImageConfigDigestKey] = imageConfigDigest.String()
+	}
+	if descriptor.MediaType != "" && descriptor.Digest.Validate() == nil && descriptor.Size > 0 {
+		metadata[docker.ExporterImageDescriptorKey] = descriptor
+	}
+	if descriptor.Digest.Validate() == nil {
+		metadata[docker.ExporterImageDigestKey] = descriptor.Digest
+	}
+	return metadata, nil
+}