Option to disable iptable REJECT target

New UFW_DISABLE_IPTABLES_REJECT option that hacks ufw to allow the
prevention of the use of the REJECT iptables target, as this is not
available on some NAS platforms (such as the Synology).

Dockerfile

@@ -116,6 +116,7 @@ ENV OPENVPN_USERNAME=**None** \
     ENABLE_UFW=false \
     UFW_ALLOW_GW_NET=false \
     UFW_EXTRA_PORTS= \
+    UFW_DISABLE_IPTABLES_REJECT=false \
     TRANSMISSION_WEB_UI= \
     PUID= \
     PGID= \