Document usage of k6 new command when used in the context of Docker

[oleiade]

A k6 user recently reported to us that they experienced a permission denied error when running k6 new with Docker: grafana/k6#3643

The solution that was offered by the team was to use the --user $UID argument as part of the docker command, and the user reported this addresses the issue they are experiencing.

As part of a follow-up, we would like to document this behavior and solution explicitly as described in this comment.

⚠️ We believe this is an issue that might impede the getting started experience of our users, and judge that it should be addressed in the documentation as soon as possible to ensure the best developer experience.