Solver: disable inline cache export for nydus compression type

Since the nydus image layer does not correspond to the cache records,
the inline cache export of nydus image is disabled here.

Signed-off-by: Yan Song <imeoer@linux.alibaba.com>

docs/nydus.md

@@ -25,6 +25,7 @@ buildctl build ... \
 
 - The export of Nydus image and runtime (e.g. [docker](https://github.com/dragonflyoss/image-service/tree/master/contrib/docker-nydus-graphdriver), [containerd](https://github.com/containerd/nydus-snapshotter), etc.) is currently only supported on linux platform.
 - Nydus image layers cannot be mixed with other compression types in the same image, so the `force-compression=true` option is automatically enabled when exporting both Nydus compression type and other compression types.
+- Since the Nydus image layer does not correspond to the cache records, the inline cache export of Nydus image is unsupported.
 - Specifying a Nydus image as a base image in a Dockerfile is not currently supported (i.e. lazily pull using a Nydus image).
 
 ### Other ways to create Nydus images

solver/llbsolver/solver.go

@@ -296,6 +296,12 @@ func inlineCache(ctx context.Context, e remotecache.Exporter, res solver.CachedR
 	if efl, ok := e.(interface {
 		ExportForLayers(context.Context, []digest.Digest) ([]byte, error)
 	}); ok {
+		if compressionopt.Type == compression.NydusBlob {
+			// Since the nydus image layer does not correspond to the cache records,
+			// the inline cache export of nydus image is disabled here.
+			return nil, errors.Errorf("inline cache with %s compression is unsupported", compressionopt.Type)
+		}
+
 		workerRef, ok := res.Sys().(*worker.WorkerRef)
 		if !ok {
 			return nil, errors.Errorf("invalid reference: %T", res.Sys())