.git and versioning in docker image

[wyli]

• the .git could be removed to reduce image size and accelerate building of the docker image
• the current image includes the version info computed from .git and not accurate:

docker run --gpus all --rm -ti --ipc=host --net=host  projectmonai/monai:0.5.0rc2
python -c "import monai; monai.config.print_config()"

MONAI version: 0.5.0rc2+0.g501026b.dirty
Numpy version: 1.19.2
Pytorch version: 1.8.0a0+52ea372
MONAI flags: HAS_EXT = True, USE_COMPILED = False
MONAI rev id: 501026ba11971eb8bbc49706a75f65ecb9e770a4

Optional dependencies:
Pytorch Ignite version: 0.4.4
Nibabel version: 3.2.1
scikit-image version: 0.15.0
Pillow version: 8.2.0
Tensorboard version: 1.15.0+nv
gdown version: 3.12.2
TorchVision version: 0.9.0a0
ITK version: 5.1.2
tqdm version: 4.53.0
lmdb version: 1.1.1
psutil version: 5.8.0

For details about installing the optional dependencies, please visit:
    https://docs.monai.io/en/latest/installation.html#installing-the-recommended-dependencies