only accept stdin if args are not passed, fix stdin hang in docker

archivebox/cli/archivebox_add.py

@@ -75,7 +75,11 @@ def main(args: Optional[List[str]]=None, stdin: Optional[IO]=None, pwd: Optional
     )
     command = parser.parse_args(args or ())
     urls = command.urls
-    stdin_urls = accept_stdin(stdin)
+
+    stdin_urls = ''
+    if not urls:
+        stdin_urls = accept_stdin(stdin)
+
     if (stdin_urls and urls) or (not stdin and not urls):
         stderr(
             '[X] You must pass URLs/paths to add via stdin or CLI arguments.\n',