osbuild-mpp: extract is_manifest_list() function

Extract the is_manifest_list() function from the ImageManifest object in
osbuild-mpp into a util function to be reused by the skopeo source.

osbuild/util/containers.py

@@ -0,0 +1,14 @@
+def is_manifest_list(data):
+    """Inspect a manifest determine if it's a multi-image manifest-list."""
+    media_type = data.get("mediaType")
+    #  Check if mediaType is set according to docker or oci specifications
+    if media_type in ("application/vnd.docker.distribution.manifest.list.v2+json",
+                      "application/vnd.oci.image.index.v1+json"):
+        return True
+
+    # According to the OCI spec, setting mediaType is not mandatory. So, if it is not set at all, check for the
+    # existence of manifests
+    if media_type is None and data.get("manifests") is not None:
+        return True
+
+    return False

tools/osbuild-mpp

@@ -322,6 +322,7 @@ import hawkey
 import rpm
 import yaml
 
+from osbuild.util import containers
 from osbuild.util.rhsm import Subscriptions
 
 # We need to resolve an image name to a resolved image manifest digest
@@ -407,13 +408,7 @@ class ImageManifest:
         self.digest = "sha256:" + hashlib.sha256(raw).hexdigest()
 
     def is_manifest_list(self):
-        #  Check if mediaType is set according to docker or oci specifications
-        if self.media_type in ("application/vnd.docker.distribution.manifest.list.v2+json", "application/vnd.oci.image.index.v1+json"):
-            return True
-        # According to the OCI spec, setting mediaType is not mandatory. So, if it is not set at all, check for the existance of manifests
-        if self.media_type == "" and self.json.get("manifests") is not None:
-            return True
-        return False
+        return containers.is_manifest_list(self.json)
 
     def _match_platform(self, wanted_arch, wanted_os, wanted_variant):
         for m in self.json.get("manifests", []):