I have a PermissionError: [Errno 13] Permission denied

Grocker does not ask for superuser rights before invoking Docker. The Docker socket has to be readable and writable by the current user. Many distributions create this socket to be readable and writable by root and the docker group.

One way to be able to use Grocker is to add your user to the docker group. On Debian:

$ sudo adduser $(whoami) docker
$ su $(whoami)  # reload groups, you should also restart your session