Support configuring the user environment

With the adoption of the j[upyter/minimal-notebook](https://jupyter-docker-stacks.readthedocs.io/en/latest/using/selecting.html#jupyter-minimal-notebook) as the base image for this stack, the user name can be controlled via the [`NB_USER` environment variable](https://jupyter-docker-stacks.readthedocs.io/en/latest/using/common.html#user-related-configurations) in combination with running the container as root user.

However, this function is currently not supported in the current `edge` version, likely because of [this documented bug](https://github.com/jupyter/docker-stacks/issues/975) that was apparently fixed in more recent versions of the Jupyter stack. We are currently forced to depend on older version due to dependency conflicts.