replace docker with dockerd

[aroraakshit]

with docker, we run into the following issue with enroot:

ubuntu@ip-172-31-5-178:~/nephele$ enroot import docker://ubuntu
[INFO] Querying registry for permission grant
[INFO] Authenticating with user: <anonymous>
[INFO] Authentication succeeded
[INFO] Fetching image manifest list
[INFO] Fetching image manifest
[ERROR] URL https://registry-1.docker.io/v2/library/ubuntu/manifests/latest returned error code: 404 Not Found

[aroraakshit]

@lukeyeager please review

[3XX0]

See NVIDIA/enroot#148
dockerd is using docker which is not installed by default

[aroraakshit]

sounds good, closing the PR