Support Docker secrets for sensitive data, e.g. `RELAYHOST_PASSWORDMAP`

Currently passwords are stored in env variable `RELAYHOST_PASSWORDMAP`.

So I think it would be nice if the container either asks for `RELAYHOST_PASSWORDMAP` or `RELAYHOST_PASSWORDMAP_FILE`. Then I could set `RELAYHOST_PASSWORDMAP_FILE=/run/secrets/relay-password` and the container could make sure that `postmap` is called on that file.

