rsync is usually a smarter choice for syncing code from the host machine to a directory inside a VM compared to scp, and is a lot performant than relying on an NFS share to sync files.

rsync --recursive --delete --relative \
    --rsh "ssh -i $DOCKER_CERT_PATH/id_rsa" \
    code/ docker@$(docker-machine ip default)/opt/volumes/my_service/

The key concept here is the use of the --rsh option, which allows you to specify the proper SSH command using the necessary identity file. DOCKER_CERT_PATH is an environment variable that gets set when we do eval $(docker-machine env default) to get Docker commands to work against the Docker daemon on the boot2docker instance.

See man rsync and Docker Machine