Update Ruby to 2.6.5

When I saw that GDK updated from 2.6.3 > 2.6.5, I realized we needed an update in this repo as well.

But... it made me wonder, why do we list the version at all in our README, whether we should instead specify something like "latest stable version".

But there's an argument for explicitly specifying the version number -- I think it's good to check that everything still works for different versions.

I notice that the version number also exists in dockerfiles/Dockerfile.gitlab-docs-base. I see @axil was the last person to change this file, so my instinct is to ask for your review. Since I see 2.6.5-alpine3.10 in https://hub.docker.com/_/ruby, I'm proposing updating the entry in the noted file accordingly. The Docker commands in https://docs.gitlab.com/ee/development/documentation/site_architecture/release_process.html seem to work (though I can't figure out how to review the output of these Docker commands).