support specifying a non-latest tag using --import-cache type=local

https://github.com/moby/buildkit/issues/1242

Signed-off-by: Tomohiro Kusumoto <zabio1192@gmail.com>

README.md

@@ -357,6 +357,7 @@ The directory layout conforms to OCI Image Spec v1.0.
 -   `ref=docker.io/user/image:tag`: reference for `registry` cache importer
 -   `src=path/to/input-dir`: directory for `local` cache importer
 -   `digest=sha256:deadbeef`: digest of the manifest list to import for `local` cache importer.
+-   `tag=customtag`: custom tag of image for `local` cache importer.
     Defaults to the digest of "latest" tag in `index.json`
 
 ### Consistent hashing

client/solve.go

@@ -423,13 +423,13 @@ func parseCacheOptions(opt SolveOpt) (*cacheOptions, error) {
 					continue
 				}
 				for _, m := range idx.Manifests {
-					if m.Annotations[ocispec.AnnotationRefName] == "latest" {
+					if (m.Annotations[ocispec.AnnotationRefName] == "latest" && attrs["tag"] == "") || (attrs["tag"] != "" && m.Annotations[ocispec.AnnotationRefName] == attrs["tag"]) {
 						attrs["digest"] = string(m.Digest)
 						break
 					}
 				}
 				if attrs["digest"] == "" {
-					return nil, errors.New("local cache importer requires either explicit digest or \"latest\" tag on index.json")
+					return nil, errors.New("local cache importer requires either explicit digest, \"latest\" tag or custom tag on index.json")
 				}
 			}
 			contentStores["local:"+csDir] = cs