Update to 8.3.0 and small optimizations

Dockerfile

@@ -2,29 +2,27 @@ FROM heroku/cedar:14
 
 MAINTAINER Stefano Sala <stefano@conversio.com>
 
-# The following setup is inspired by the Heroku Node-js dockerfile with some
-# extras and a different node version.
+WORKDIR /app/user
+
+# The following setup is inspired by the Heroku Node-js dockerfile with some extras and a different node version.
 
 # Which version of node?
-ENV NODE_ENGINE 7.10.1
+ENV NODE_ENGINE 8.3.0
 
 # Locate our binaries
 ENV PATH /app/heroku/node/bin/:/app/user/node_modules/.bin:$PATH
 
 # Create some needed directories
 RUN mkdir -p /app/heroku/node /app/.profile.d
-WORKDIR /app/user
 
 # Install node
-RUN curl -s https://s3pository.heroku.com/node/v$NODE_ENGINE/node-v$NODE_ENGINE-linux-x64.tar.gz | tar --strip-components=1 -xz -C /app/heroku/node
-
-# Export the node path in .profile.d
-RUN echo "export PATH=\"/app/heroku/node/bin:/app/user/node_modules/.bin:\$PATH\"" > /app/.profile.d/nodejs.sh
+RUN curl -s https://s3pository.heroku.com/node/v$NODE_ENGINE/node-v$NODE_ENGINE-linux-x64.tar.gz | tar --strip-components=1 -xz -C /app/heroku/node \
+  && echo "export PATH=\"/app/heroku/node/bin:/app/user/node_modules/.bin:\$PATH\"" > /app/.profile.d/nodejs.sh
 
 # Install yarn
-RUN curl -sS http://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
-RUN echo "deb http://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
-RUN apt-get update && apt-get install -y yarn
+RUN curl -sS http://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
+  && echo "deb http://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list \
+  && apt update && apt install -y yarn && rm -rf /var/lib/apt/lists/* /var/cache/apt/*
 
 # Replace shell with bash so we can source files
 RUN rm /bin/sh && ln -s /bin/bash /bin/sh