Fallback to host architecture if image architecture is not set

There can be cases, where the architecture of an image is not set, even
though both the Docker Image spec and OCI Image spec require this field.
A nice workaround is to fallback to the platform of the underlying host,
the container running was observed. This will probably not be 100%
correct in all cases, but this would make it an edge case of an edge
case, meaning it is good enough.

cmd/imagescan/collector/collector.go

@@ -95,11 +95,18 @@ func (c *Collector) Collect(ctx context.Context) error {
 		return fmt.Errorf("extract manifest digest: %w", err)
 	}
 
+	arch := arRef.ConfigFile.Architecture
+	// There are rare cases where the Architecture of an image is not set (even though this doesn't appear to be
+	// neither OCI nor docker image spec conform). This is not a 100% solution, but should be good enough.
+	if strings.TrimSpace(arch) == "" {
+		arch = c.hostFsConfig.Platform.Architecture
+	}
+
 	metadata := &castaipb.ImageMetadata{
 		ImageName:    c.cfg.ImageName,
 		ImageId:      c.cfg.ImageID,
 		ImageDigest:  digest.String(),
-		Architecture: arRef.ConfigFile.Architecture,
+		Architecture: arch,
 		ResourceIds:  strings.Split(c.cfg.ResourceIDs, ","),
 	}
 	if arRef.OsInfo != nil {