Support Docker volumes

[vincenzobaz]

When I try to use a Docker volume name as argument for grade local or for inspect -d I receive the error

courseraprogramming inspect: error: argument -d/--submission: XX is not a directory
courseraprogramming grade local: error: argument dir: XX is not a directory

This is a limitation when CI jobs are executed inside containers.

docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock -v VOLUME:/subz ci-image /bin/ash
docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock -v VOLUME:/subz courseraprogramming grade local 8f0bcd3e9eb8 VOLUME # error
docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock -v VOLUME:/subz courseraprogramming grade local 8f0bcd3e9eb8 /subz # files are nout mounted