Update Dockerfile to use Ruby >= 2.6 in the tool to preview Geode documentation

The script to preview the documentation of Geode (./preview-user-guide.sh) is not working anymore.

The following error appears while running it (unless you have a previously downloaded geodedocs/temp docker image in your local docker repo):

ERROR: Error installing elasticsearch:
The last version of faraday (>= 0) to support your Ruby & RubyGems was 1.10.0. Try installing it with `gem install faraday -v 1.10.0` and then running the current command again
faraday requires Ruby version >= 2.6. The current ruby version is 2.5.9.229.

That error prevents the preview script to work.

It is needed to update the docker image used to preview the documentation to use a Ruby version >= 2.6