Added CerebNet, removed copying empty dir and corrected ckpt downloaddir

[LeHenschel]

Inference with Dockerfiles_FastSurferCNN and _CPU failed because ckpts were not downloaded correctly during the build phase. In the Dockerfile, the checkpoint dir (which no longer exists) was copied, and the download was not run at all. During the docker run, attempts to download the checkpoints failed due to
1.) No write access to the directory
2.) CerebNet directory missing (module could hence not be loaded)

Both points are fixed by removing the checkpoints copy command and adding the cerebnet copy command.

Tested for CPU and GPU on one image. AMD was not tested, as I do not have access to a compatible computer.