Use stable expat to fix CVE-2016-4472

ejabberd-mix/Dockerfile

@@ -4,12 +4,13 @@ MAINTAINER ProcessOne <contact@process-one.net>
 RUN echo \
   # Replacing default repositories with edge ones as we need Elixir 1.3.x
   # Previous version of Elixir do not handle our deps properly
-  && echo "http://dl-cdn.alpinelinux.org/alpine/edge/testing" > /etc/apk/repositories \
+  && echo @stable "http://dl-cdn.alpinelinux.org/alpine/v3.4/main" > /etc/apk/repositories \
+  && echo "http://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories \
   && echo "http://dl-cdn.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories \
   && echo "http://dl-cdn.alpinelinux.org/alpine/edge/main" >> /etc/apk/repositories && \
 
   # Install elixir from apk
-  apk add --no-cache build-base git zlib-dev openssl-dev yaml-dev expat-dev \
+  apk add --no-cache build-base git zlib-dev openssl-dev yaml-dev expat-dev@stable \
    elixir erlang-crypto erlang-syntax-tools erlang-parsetools \
    erlang-eunit erlang-mnesia