Add some information about the use of the -u switch when using the Docker Singularity container

[jcohen02]

When running the Docker Singularity container, we encounter an issue with the generated images stored in a bind-mounted directory being owned by root on the user's local system when they've exited from the container.

As highlighted in carpentries-incubator/docker-introduction#65, we should add something into this material to highlight the use of the -u switch rather than the current quick workaround suggestion to take a copy of the image to get a version with user ownership.

This update should be made in the "Permissions of the created image file" callout in _episodes/03-singularity-images.md