Mention Redmine plugins that depend on gem native extensions

[delanym]

Plugins are an integral part of Redmine. A number of issues have been reported about plugins that require native extensions:
https://github.com/docker-library/redmine/issues?q=is%3Aissue+native+extensions

If a plugin is installed into a container of the standard image the application exits without a log line to indicate why.

It would be a great help if the documentation for these images could acknowledge that they may not be enough to support plugins that depend on gems that include native extensions.

It would also help to have an example of docker-library/redmine#331 (comment) for Debian, Alpine, etc.