mailcow: Fix netfilter Dockerfile.

Change-Id: I1ffc7ff5314335eee642777dfaf3b14390a8c129
diff --git a/mailcow/src/mailcow-dockerized/data/Dockerfiles/netfilter/Dockerfile b/mailcow/src/mailcow-dockerized/data/Dockerfiles/netfilter/Dockerfile
index c63e99b..14b3464 100644
--- a/mailcow/src/mailcow-dockerized/data/Dockerfiles/netfilter/Dockerfile
+++ b/mailcow/src/mailcow-dockerized/data/Dockerfiles/netfilter/Dockerfile
@@ -16,6 +16,8 @@
   tzdata \
   py3-pip \
   musl-dev \
+&& pip3 install --upgrade --ignore-installed \
+  packaging \
 && pip3 install --upgrade pip \
   python-iptables \
   redis \