Docker: add options for multicast groups (#290)

Dockerfile

@@ -25,6 +25,7 @@ ENV UDP_TIMEOUT=
 ENV TCP_SNDBUF=
 ENV TCP_RCVBUF=
 ENV TCP_AUTO_TUNING=
+ENV MULTICAST_GROUPS=
 ENV EXTRA_COMMANDS=
 ENV TUN_INCLUDED_ROUTES=
 ENV TUN_EXCLUDED_ROUTES=

docker/entrypoint.sh

@@ -81,6 +81,10 @@ run() {
     ARGS="$ARGS --tcp-auto-tuning"
   fi
 
+  if [ -n "$MULTICAST_GROUPS" ]; then
+    ARGS="$ARGS --multicast-groups $MULTICAST_GROUPS"
+  fi
+
   exec tun2socks \
     --loglevel "$LOGLEVEL" \
     --fwmark "$FWMARK" \