Update Dockerfile to be able to build 17.04 ejabberd version

Because the default installation prefix has changed in the new release, it is needed to add in the configure line of Docker file the argument  --prefix=/

Dockerfile

@@ -1,7 +1,7 @@
 FROM debian:jessie-slim
 MAINTAINER Rafael Römhild <rafael@roemhild.de>
 
-ENV EJABBERD_BRANCH=17.03 \
+ENV EJABBERD_BRANCH=17.04 \
     EJABBERD_USER=ejabberd \
     EJABBERD_HTTPS=true \
     EJABBERD_STARTTLS=true \
@@ -68,6 +68,7 @@ RUN set -x \
     && chmod +x ./autogen.sh \
     && ./autogen.sh \
     && ./configure --enable-user=$EJABBERD_USER \
+        --prefix=/ \
         --enable-all \
         --disable-tools \
         --disable-pam \