WIREGUARD_PRIVATE_KEY_SECRETFILE missing

[Mladia]

https://github.com/qdm12/gluetun/blob/c5cc240a6cf8b97de8bcf199b8ad23a49deb2b48/Dockerfile#L96C19-L96C19

Currently, the possibility of using a secrets file for Wireguard is missing.
This functionality is present for instance with Openvpn and the OPENVPN_USER_SECRETFILE.

As I understand, one of the things is to add
WIREGUARD_PRIVATE_KEY_SECRETFILE=/run/secrets/wireguard_private_key \.
to the Dockerfile

Would this be all?

[qdm12]

Duplicate of #614

Also see #1348 (comment) and feel free to subscribe to the issue or pr mentioned to be alerted once it's done. Thanks!