[docker] Combine apt-get update with apt-get install.

Running RUN apt-get update by itself will cause it to use the cache when
rebuilding the dockerfile. This can lead to apt-get installs failing
when updating programs. The preferred method is RUN apt-get update &&
apt-get install -y <any apt-get installs you need>. this will force
apt-get update to run when any package is updated, or new ones are
installed.

moby/moby#3313

Dockerfile

@@ -1,17 +1,17 @@
 FROM python:2.7
 ADD . /code
 WORKDIR /code
-RUN apt-get update
+
+# update apt-get and install packages
+RUN apt-get update && apt-get install -y locales nodejs npm
 
 # Generate locales
-RUN apt-get install -y locales
 RUN echo "en_US.UTF8 UTF-8" > /etc/locale.gen
 RUN locale-gen
 
 # Node dependencies
-RUN apt-get install -y nodejs npm
 RUN npm install -g less
 
 RUN pip install -r build-requirements.txt
 RUN ln -s /usr/bin/nodejs /usr/bin/node
-RUN peep install -r requirements/dev.txt
+RUN peep install -r requirements/dev.txt