Issue #3093996 by heddn: Opcode cache checker is not registered to run

ReadinessCheckers/ReadinessCheckerManager.php

@@ -29,6 +29,7 @@ class ReadinessCheckerManager {
     static::$checkers['warning'][0][] = 'ModifiedFiles';
     static::$checkers['warning'][0][] = 'PhpSapi';
     static::$checkers['error'][0][] = 'DiskSpace';
+    static::$checkers['error'][0][] = 'OpcodeCache';
     static::$checkers['error'][0][] = 'PendingDbUpdates';
     static::$checkers['error'][0][] = 'ReadOnlyFilesystem';