systemd-247 provides a mechanism called LoadCredential for secrets and
it is better than environment file. See the section of Environment=
in the manual of systemd.exec for more information.
Some options in config.yaml need values to be strings, which currently
can be used with environmentFile but not loadCredential. But it's
possible to use loadCredential for those options, e.g. we can
substitute their values in ExecStart, but not in ExecStartPre due to
[1].
[1]: https://github.com/systemd/systemd/issues/19604
