The `$(</path/to/file)`-expansion appears verbatim in the cmdline of `nextcloud-occ` which means that an unprivileged user could find sensitive values (i.e. admin password & database password) by monitoring `/proc/<pid>/cmdline`. Now, these values don't appear in a command line anymore, but will be passed as environment variables to `nextcloud-occ`.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at https://nixos.org/nixos and in the manual in doc/manual.