Non-root SSH key authentication problems with DSM 6

With Synology DSM 6 it seems that SSH requires users to be members of the admin group. However, even in this case there seem to be problems with file/directory permissions that prevent key-based authentication succeeding for non-root users (standard /root/.ssh/authorized_keys works OK for root). Specifically, SSH key-based authentication fails even with a correct ~/.ssh/authorized_keys in place.