update.sh does not update example version

[mvarendorff]

I don't know if that is in scope for the script but I noticed in the ruby docs that the example states FROM ruby:2.5 which isn't listed as tag in the list at the top of the docs.

This should ideally be covered by automated updates as well.

[tianon]

Yeah, we've talked quite a few times about automating those types of version bumps, but it's very hard to generically determine which version is the "latest" or "best" version to list in the templates, unfortunately.