assign empty slice if local digests is nil

Author: satotake

pkg/compose/pull.go

@@ -209,7 +209,10 @@ func (s *composeService) pullDryRun(ctx context.Context, project *types.Project,
 	var results []pullDryRunServiceResult
 
 	for _, service := range project.Services {
-		l := localDigests[service.Image]
+		l, ok := localDigests[service.Image]
+		if !ok {
+			l = []string{}
+		}
 		d := dstrDigests[service.Image]
 		plan := getPullPlan(service, l, d)
 		result := &pullDryRunServiceResult{