Clarify root user usage for rstudio when rootless

[zeehio]

Closes rocker-org/rocker-versioned2#844

Thanks to @Tianmaru for finding and reporting the issue

[eitsupi]

Thanks for working on this.

I have never used this container with rootless, so I can't determine if this information is correct without knowing how it actually works.
If I log into RStudio as root, what do I enter as the password?

[zeehio]

@eitsupi, I just added a commit explicitly stating that you should use the password displayed in the console.

I hope this answers the question

[Tianmaru]

@zeehio @eitsupi I think it's clear to you, but just to make sure: If I start the container in rootless mode, I can still use -e PASSWORD=<pw>, to specify the password I want to login as user root, instead of relying on the randomly generated password.

[eitsupi]

Thanks both, that sounds great!
Let's merge for now.