Make it more obvious how to run a node image as non-root

[rugk]

Description

This IMHO highlights a critical part.

Motivation and Context

Fixes https://github.com/nodejs/docker-node/issues/2287 (see there for more reasoning)

IMHO, you may also link the best practices on https://hub.docker.com/_/node/#how-to-use-this-image directly, to save some more clicks and hopefully make more people find the information about non-root users.

I personally have not found that information as I searched for best on docker-hub and for root there and this Readme. (I missed searching for best again so maybe this helps.)

Testing Details

Example Output(if appropriate)

Types of changes

• [x] Documentation
• [ ] Version change (Update, remove or add more Node.js versions)
• [ ] Variant change (Update, remove or add more variants, or versions of variants)
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Other (none of the above)

Checklist

• [x] My code follows the code style of this project.
• [ ] ~~My change requires a change to the documentation.~~
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING.md document.
• [x] All new and existing tests passed.