Use Alpine based image for app (#208)

app/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM alpine:3.6
 
 # Some ENV variables
 ENV PATH="/mattermost/bin:${PATH}"
@@ -8,15 +8,17 @@ ENV MM_VERSION=4.6.1
 ARG edition=enterprise
 
 # Install some needed packages
-RUN apt-get update \
+RUN apk add --no-cache \
-    && apt-get -y --no-install-recommends install \
+	ca-certificates \
-      curl \
+	curl \
-      jq \
+	jq \
-      netcat \
+	libc6-compat \
-      ca-certificates \
+	libffi-dev \
-      xmlsec1 \
+	linux-headers \
-      mime-support \
+	mailcap \
-    && rm -rf /var/lib/apt/lists/* /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin
+	netcat-openbsd \
+	xmlsec-dev \
+	&& rm -rf /tmp/*
 
 # Get Mattermost
 RUN mkdir -p /mattermost/data \

app/entrypoint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 # Function to generate a random salt
 generate_salt() {