Re-order build process for onbuild, aptfile

[doprdele]

Describe the bug

Packrat libraries are installed first, rather than after onbuild and aptfile. This makes installing rJava impossible.

To Reproduce

Add an onbuild with

#!/bin/bash
# install java jdk
apt-get update -q
apt-get install -qy --no-install-recommends openjdk-8-jdk openjdk-8-jre
rm -rf /var/lib/apt/lists/*

# configure R for Java
R CMD javareconf &> /dev/null

to the example and try to build it.

Expected behavior

Docker container builds and rJava is installed.

[doprdele]

This is ready for QA. If you follow the example in the documentation for rJava here https://hmdc.gitbook.io/r/config/addsystempackages, the sample heroku docker app should work.

Steps to QA:

• Following the above instructions, modify the https://github.com/hmdc/heroku-docker-r-example-app repository by adding rJava.
• It should deploy, work, and be accessible.

If testing locally, you will need to
docker pull hmdc/heroku-docker-r:3.6.1-shiny
to get the latest container image.

[mreekie]

The docker container for R allows ou to run custom code at the beginnning.
Figures people will be installing Java at this point. This is a system package.
The issue is that any system packages like java need to be installed prior to installing any R library packages. This was not happening. This issue addresses this.