Serbero v0.2.1

Added

• Secrets can be read from files: SERBERO_PRIVATE_KEY_FILE and
  TYPESAFE_API_KEY_FILE (any *_env variable with _FILE appended), as
  Docker secrets and systemd credentials provide. Setting both forms is an
  error, and a secret file readable by any user is logged as a warning.

Changed

• deploy/compose.yml passes secrets as Docker secrets and
  deploy/serbero.service as systemd credentials, so they no longer sit in
  the container or process environment. serbero.env now holds only
  non-secret settings.