Gitlab-CI: Run CI an docker image and checks organisation

Using docker

Currently the checks are performed on a server hosted by Inrae at CINES Montpellier.

I run directly the checks in the environment of the server and I have manually compiled 3 versions of R:

oldrel: 3.6.3
patched: 4.0.5
devel: 2020-04-28 r78328

We can see that these version are outdated and maintaining these version updated manually is a bit time-consuming.

Using docker images allows to always work with up-to-date versions of R and moreover allows to use the dedicated Gitlab-runner server managed by Inrae.

On the rocker/tidyverse repository, we can use the following versions (See https://hub.docker.com/r/rocker/tidyverse/tags):

latest (which correspond today to the 4.1.1 "patched" version).
devel

There is no alias for "old-rel", but we can manage it manually by picking up an old version we want to assure compatibility (for example rocker/tidyverse:3.5).

Check organisation

Checks "on-cran" and checks "not-cran" are partially redundant. Maybe we can only run checks on "cran" and the totality of the tests to save running time.