Improve clarity and build process

[andrewnicols]

We should improve our Dockerfile in line with best-practice recommendations in https://github.com/docker-library/official-images/blob/master/README.md#clarity

Specifically we should move all of the commands out of the ADD/RUN syntax and into the main Dockerfile.

We should also look to improve our build process.
I would recommend we look at what PHP, Postgres, and others have done and to have a shell script (update.sh) which takes a matrix of PHP versions, Base image versions (stretch at this time), and generates a Dockerfile in a folder format for each.

[stronk7]

Maybe this can be closed now with all the recent updates performed?