feat(config): read Wireguard config from secret

- defaults to `/run/secrets/wg0.conf`
- can be changed with variable `WIREGUARD_CONF_SECRETFILE`

internal/configuration/sources/secrets/wireguard.go

@@ -0,0 +1,21 @@
+package secrets
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/configuration/sources/files"
+)
+
+func (s *Source) readWireguard() (settings settings.Wireguard, err error) {
+	wireguardConf, err := s.readSecretFileAsStringPtr(
+		"WIREGUARD_CONF_SECRETFILE",
+		"/run/secrets/wg0.conf",
+	)
+	if err != nil {
+		return settings, fmt.Errorf("reading Wireguard conf secret file: %w", err)
+	} else if wireguardConf != nil {
+		return files.ParseWireguardConf([]byte(*wireguardConf))
+	}
+	return settings, nil
+}