Docker (best practices) docs recommending usage of --system

[TaylorZowtuk]

@matteius I just wanted to clarify whether using --system is really a best practice as indicated in the Docker best practices docs (since #6379). In that PR, the examples all use --system and it is explicitly included in the 'Best Practices' summary at the end of the doc:

5. Install dependencies system-wide with --system to avoid unnecessary virtual environments

In #2762, it was previously decided to recommend not using --system in Docker. I wonder if there was something incorrect about the discussions and conclusions in that PR, or if things have changed since then - and if so, in what ways?

Thanks for your time!