Fix Dockerfile

[fixdocker]

Pull Request (PR) description

Hello, I represent a research group investigating build failures in
Dockerfiles. We executed a Dockerfile in your project and found that
it is not building. We are sending this Pull Request with a suggested
repair. The repair operation is based the following URL contextualized to this case:

• Change locale to c.utf-8 docker-library/ruby#45

Please note that we are using an anonymous GitHub account because of
double-blind restrictions in the submission process of conferences.

This Pull Request (PR) fixes the following issues

[fixdocker]

Hey @bastelfreak @dickp , can you check this PR?

[bastelfreak]

Hello,
please see the full thread here: https://twitter.com/BastelsBlog/status/1295395255383207937
I don't think that this fix is the correct approach. Instead the base image should be updated to the correct ruby one. Also our dockerfile, where you send us multiple PRs, comes from a central place: https://github.com/voxpupuli/modulesync_config/blob/master/moduleroot/Dockerfile.erb
Please do not provide patches for our 125 other identical dockerfiles

[fixdocker]

hello @bastelfreak, we apologize for any problems that this and the other
pull requests have caused, our intention is just to help.

What do you think of us sending a single PR to the central
place, just updating the version of the base image to 2.5.8
as mentioned in the twitter thread?