Update tensorflow-cpu to tensorflow 2.2

[FvD]

The current image is still using R 3.6 and TF 1.2. it it still works for many examples but we are unable to save_model_tf().

[cboettig]

Thanks for the ping, the current images are actually now over on rocker-org/rocker-versioned2, and include: rocker/ml rocker/ml-verse They have cuda10.1 (compatible with 11.1 drivers) and more recent tensorflow etc. feedback welcome!