docker-cp copy files from and to docker containers with buffers

usage: docker-cp.local [-h] [--buffer-length N] source dest

Copies files from and to containers

positional arguments:
  source             Source location
  dest               Destination location

optional arguments:
  -h, --help         show this help message and exit
  --buffer-length N  Restrict the size of the buffer used for reading/writing
                     files transfered to and from docker containers