Sounds like the same problem, i run into.
If i place the secrets json in the config directory i get following Error.
docker exec GooglePhotosSync gphotos-sync --secret /config/client_secret.json /storage
10-08 16:41:20 WARNING gphotos-sync 3.1.2 2023-10-08 16:41:20.623633
10-08 16:41:28 ERROR
Process failed.
Traceback (most recent call last):
File "/usr/lib/python3.9/site-packages/gphotos_sync/Main.py", line 506, in main
self.setup(args, db_path)
File "/usr/lib/python3.9/site-packages/gphotos_sync/Main.py", line 346, in setup
self.auth = Authorize(
File "/usr/lib/python3.9/site-packages/gphotos_sync/authorize.py", line 51, in __init__
secrets = all_json["installed"]
KeyError: 'installed'
10-08 16:41:28 WARNING Done.
I have followed the instructions.