Suggestion: Update the docker image used for the "rsyslogdoc ubuntu16 rsyslog" CI build to include the Better theme

[deoren]

The current CI builds are a very useful feature, but it would be useful to be able to see what the results will look like on the website once the PR is approved.

If wishes were fishes and all that, this request would be for another preview build to showcase both the stock theme and the website theme. It only just takes time to do these things, right? ;)