fix(Dockerfile): ensure apt-get install is prefixed by update

Since Dockerfile layers are cached, having `apt-get update` as a separate
command creates the potential for a subsequent `apt-get install` to fail.
Best practices (other than "install from source") seem to be to make
both commands into a one-liner.

This PR ensures we don't `apt-get install` as its own command anywhere,
removes the installation of some packages that were already in deis/base,
and removes a redundant ENV DEBIAN_FRONTEND statement.

Author: mboersma

Dockerfile

@@ -2,18 +2,14 @@ FROM deis/base:latest
 MAINTAINER OpDemand <info@opdemand.com>
 
 # install required system packages
+# HACK: install git so we can install bacongobbler's fork of django-fsm
+# install openssh-client for temporary fleetctl wrapper
 RUN apt-get update && \
-    apt-get install -yq python-dev libpq-dev libyaml-dev
+    apt-get install -yq python-dev libpq-dev libyaml-dev git openssh-client
 
 # install recent pip
 RUN wget -qO- https://raw.githubusercontent.com/pypa/pip/1.5.5/contrib/get-pip.py | python -
 
-# HACK: install git so we can install bacongobbler's fork of django-fsm
-RUN apt-get install -yq git
-
-# install openssh-client for temporary fleetctl wrapper
-RUN apt-get install -yq openssh-client
-
 # add a deis user that has passwordless sudo (for now)
 RUN useradd deis --groups sudo --home-dir /app --shell /bin/bash
 RUN sed -i -e 's/%sudo\tALL=(ALL:ALL) ALL/%sudo\tALL=(ALL:ALL) NOPASSWD:ALL/' /etc/sudoers