Images Ubuntu 16.04, optimization and reduce image size (#204)

app/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:14.04
+FROM ubuntu:16.04
 
 # Some ENV variables
 ENV PATH="/mattermost/bin:${PATH}"
@@ -9,10 +9,11 @@ ARG edition=entreprise
 
 # Install some needed packages
 RUN apt-get update \
-    && apt-get -y install \
+    && apt-get -y --no-install-recommends install \
       curl \
       jq \
       netcat \
+      ca-certificates \
     && rm -rf /var/lib/apt/lists/* /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin
 
 # Get Mattermost