docker: mv r folder to r-base folder to be consistent with img names

src/.docker_modules/r-base/3.5.3/Dockerfile

src/.docker_modules/r-base/3.5.3/docker_init.sh

src/.docker_modules/r-base/3.6.2/Dockerfile

src/.docker_modules/r-base/3.6.2/docker_init.sh

src/.docker_modules/r-base/4.0.0/Dockerfile

src/.docker_modules/r-base/4.0.0/docker_init.sh

src/.docker_modules/r-base/4.0.2/Dockerfile

src/.docker_modules/r-base/4.0.2/docker_init.sh