Currently there is rocker/verse:devel under CircleCI testing. However this version is not a stable version, and CircleCI may fail because of this. To support the following R versions in CircleCI testing:

• [FIX] CircleCI supports R 3.6 as it is a popular R version #51
• R 4.1.0, to fix the issue described in [FIX] releaseR4.1.0 - error in CircleCI in branch: enh/benchmark_lm #24