Address CR comments

ci/build.py

@@ -90,16 +90,6 @@ def under_ci() -> bool:
     return 'JOB_NAME' in os.environ
 
 
-def git_cleanup() -> None:
-    """Clean repo and subrepos, update subrepos"""
-    logging.info("cleaning up repository")
-    with remember_cwd():
-        os.chdir(get_mxnet_root())
-        check_call(['git', 'clean', '-ffdx'])
-        check_call(['git', 'submodule', 'foreach', '--recursive', 'git', 'clean', '-ffdx'])
-        check_call(['git', 'submodule', 'update', '--recursive', '--init'])
-
-
 def get_dockerfiles_path():
     return "docker"
 
@@ -148,6 +138,10 @@ def build_docker(platform: str, docker_binary: str, registry: str, num_retries:
     # cache-from is needed so we use the cached images tagged from the remote via
     # docker pull see: docker_cache.load_docker_cache
     #
+    # This also prevents using local layers for caching: https://github.com/moby/moby/issues/33002
+    # So to use local caching, we should omit the cache-from by using --no-cache argument to this
+    # script.
+    #
     # This doesn't work with multi head docker files.
     #
     cmd = [docker_binary, "build",
@@ -253,7 +247,7 @@ def container_run(platform: str,
         logging.error("You can get into the container by adding the -i option")
         raise subprocess.CalledProcessError(ret, cmd)
 
-    return ret 
+    return ret
 
 
 def list_platforms() -> str:
@@ -365,7 +359,8 @@ def main() -> int:
                         type=int)
 
     parser.add_argument("--no-cache", action="store_true",
-                        help="Enable docker registry cache")
+                        help="Disables use of --cache-from option on docker build, allowing docker"
+                        "to use local layers for caching. If absent, we use the cache from dockerhub.")
 
     parser.add_argument("command",
                         help="command to run in the container",
@@ -433,7 +428,6 @@ def use_cache():
                          use_cache=use_cache())
             if args.build_only:
                 continue
-            git_cleanup()
             shutil.rmtree(buildir(), ignore_errors=True)
             build_platform = "build_{}".format(platform)
             plat_buildir = os.path.abspath(os.path.join(get_mxnet_root(), '..',
@@ -472,8 +466,7 @@ def use_cache():
 
 ./build.py -a
 
-    Builds for all platforms and leaves artifacts in build_<platform>. **WARNING** it performs git
-    cleanup of the repo.
+    Builds for all platforms and leaves artifacts in build_<platform>.
 
     """)