[Netfilter] Use Alpine 3.6

data/Dockerfiles/netfilter/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.7
+FROM alpine:3.6
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 RUN apk add -U python2 python-dev py-pip gcc musl-dev iptables ip6tables \

docker-compose.yml

@@ -320,7 +320,7 @@ services:
             - acme
 
     netfilter-mailcow:
-      image: mailcow/netfilter:1.12
+      image: mailcow/netfilter:1.13
       build: ./data/Dockerfiles/netfilter
       stop_grace_period: 30s
       depends_on: