fix(compose): use VPN_MTU from .env instead of hardcoded 1280

Author: zerox80

backend/docker-compose.yml

@@ -21,7 +21,7 @@ services:
       - VPN_NETWORK=10.8.0.0/24
       - VPN_NETWORK_V6=fd00::/64
       - VPN_DNS=1.1.1.1
-      - VPN_MTU=1280
+      - VPN_MTU=${VPN_MTU:-1280}
       - VPN_CENSORSHIP_RESISTANT=${VPN_CENSORSHIP_RESISTANT:-false}
       - VPN_MSS_CLAMPING=${VPN_MSS_CLAMPING:-true}
       - VPN_KEYCLOAK_ENABLED=${VPN_KEYCLOAK_ENABLED}