Fix permission/ownership of created data when using docker

[daniel-wer]

When using docker to run the cuber, the created data directories and files are root-owned, which is not ideal and creates problems when further processing the data or importing it into wK. Ideally, the files should belong to the user that executes the cubing script. @jstriebel mentioned this could be achieved by wrapping the docker command in a small script to correctly set the UID/GID. Imo, it would be even better if we could achieve this without the need to have this extra script, so a user can simply copy the command and execute it.

[normanrz]

Basically you need to add -u $(id -u):$(id -g) to the docker run command. Either in the readme or in a script. There is no intrinsic way to do this.