Make R builds self contained

[gaborcsardi]

I.e. set env vars and other config for the build, and not for the whole Dockerfile.

This would allow installing multiple R builds in the same system.

This mainly means

• making sure external software is available,
• setting env vars from the system environment file or the system profile,
• setting Makeconf stuff in the system Makeconf file, not in ~/.R/Makeconf.

[gaborcsardi]

I have no time for this now.