stable build environment using docker

[mammykins]

No description provided.

[nacnudus]

Renamed to Stable build environment, as in, to build the RAP Companion. Docker in general is being discussed elsewhere.

I have mixed feelings about this.

Pros:

1. Easier to quickly update -- a simple PR was delayed for ages by packrat being difficult over a system dependency upgrade. This isn't a user need, and the delay was organisational rather than technical. Plus I learned a lot by fixing the issue.
2. Establishes a pattern for docker build environments. This need can be met independently of building the RAP Companion.

Cons:

• Build failures alert us to outdated code in the text of the book (i.e. outdated advice). Users need up-to-date advice.
• Unpatched build environments are a security risk.