[test][bcp] Fix name of docker image cache class variable

It was useless since the file path was store in an unrelated
variable.

beat/core/test/test_bcp.py

@@ -322,7 +322,7 @@ class TestBCPDocker(TestBCP):
 
     @classmethod
     def setUpClass(cls):
-        cls.images_cache = os.path.join(tmp_prefix, "docker_images_cache.json")
+        cls.docker_images_cache = os.path.join(tmp_prefix, "docker_images_cache.json")
         cls.host = Host(images_cache=cls.docker_images_cache, raise_on_errors=False)