Only override the graphdriver to vfs if the priority is unset

This is an amend to containers#1460

That PR was not addressing the case when the system wide config had the
driver_priority option configured and the user had no config file of their
own. Then `getRootlessStorageOpts` would be called and it would override the
graph driver to "vfs".

With this commit we only override the graph driver if driver priority is
empty. Otherwise we propagate the driver priority into the storage options, so
that the driver autodetection works as expected.

Signed-off-by: Dan Čermák <dcermak@suse.com>

types/options.go

@@ -300,7 +300,11 @@ func getRootlessStorageOpts(rootlessUID int, systemOpts StoreOptions) (StoreOpti
 		}
 	}
 	if opts.GraphDriverName == "" {
-		opts.GraphDriverName = "vfs"
+		if len(systemOpts.GraphDriverPriority) == 0 {
+			opts.GraphDriverName = "vfs"
+		} else {
+			opts.GraphDriverPriority = systemOpts.GraphDriverPriority
+		}
 	}
 
 	if os.Getenv("STORAGE_OPTS") != "" {