using better help message for --docker-pull flag

Author: kapilkd13

cwltool/builder.py

@@ -49,6 +49,7 @@ def __init__(self):  # type: () -> None
         self.make_fs_access = None  # type: Type[StdFsAccess]
         self.debug = False  # type: bool
         self.mutation_manager = None  # type: MutationManager
+        self.strict_docker_pull = False  # type: bool
 
         # One of "no_listing", "shallow_listing", "deep_listing"
         # Will be default "no_listing" for CWL v1.1

cwltool/main.py

@@ -227,8 +227,8 @@ def arg_parser():  # type: () -> argparse.ArgumentParser
                          help="Generate a template input object")
 
     parser.add_argument("--docker-pull", action="store_true",
-                        default=False, help="Docker pull image again even if it is"
-                                            " already present", dest="docker_pull")
+                        default=False, help="Strictly pull latest docker image even if"
+                                            " it is locally present", dest="docker_pull")
     parser.add_argument("workflow", type=Text, nargs="?", default=None)
     parser.add_argument("job_order", nargs=argparse.REMAINDER)