Enable Docker images to run shell scripts for exercise test commands

[GreatGameDota]

During running of Docker images on a physical Duckiebot the Docker image will run a shell command which runs the main script for the exercises. However during the entrypoint script Docker runs, it fails to run the script because the script wasn't given proper permission. This pull request adds a simple command to address this issue with no conflicts with the current code.

[liampaull]

I think the file itself should have the permissions changed rather than changing this in the dts command

[afdaniele]

agree, this is now fixed by changing the file permissions. Thanks for contributing.