Update for Release 3.9

[sneumann]

Hi,
rocker/rstudio:3.6.0 appeared yesterday.
For release we need a branch R3.6.0_Bioc3.9 The 3.8 release branch was missing the _ (minor cosmetic itch).

Configuration happens in https://github.com/Bioconductor/bioc_docker/blob/master/config.yml
It would be great to cleanup some of the comments and make them more explicit,
e.g. should always be false to should always be false in release, or 'true half the year->true during devel cycle may to october` (or was it the other devel cycle ?) and remove outdated comments (rstudio-daily does not exist anymore ?).

Check if we still want to support use_numbered_branch, which is used in
https://github.com/Bioconductor/bioc_docker/blob/master/src/base/Dockerfile.in#L72
to (optionally) install an R from sources. This does help during the weeks where rstudio:devel
reports the next R version, which has (and will) cause the bioc docker to fail during april,
unless the biocVersion hack is used in our Dockerfile(s). But it would be a very special case,
as we can't be sure our R compilation is consistent with what rocker uses, so I suggest to stick
with the rocker R version in our images always.

Yours,
Steffen

[lshep]

I will make some time in the upcoming weeks to clean up the notes in the config file as well as clean up the base/core installation scripts. For now moving through the pull request change so the dockers can start building the correct images/versions