Use Case:

Systemd provides the "StateDirectory=" directive which sets the $STATE_DIRECTORY environment variable to a systemd managed persistent directory (for example systemd ensures that it has the right permissions if the service file uses the DynamicUser facilities).

It would be nice to be able to use this as the storage directory for the file system storage.

Possible Implementation:

The simplest implementation enabling this use case would be to expand environment-variables in fs:path (for example via os.ExpandEnv())
Other possibilities include doing this expansion for all configuration values or handling the case of systemd explicitly as part of the fs storage driver (e.g. activly looking for $STATE_DIRECTORY and possibly $CREDENTIALS_DIRECTORY introduced in systemd 247.)

If you have a preferred solution I would be happy to open a corresponding pull request :)