Add support for publishing DNS entries of loadbalancer services

With the new LBaaS feature [1], we also need to expose the IP addresses of load balancer instances as proper DNS names.

This MR also adds Shellcheck [2] into the CI pipeline, which allows catching common shell programming mistakes and pitfalls.

[1] https://gitlab.cern.ch/webservices/webframeworks-planning/-/issues/638

[2] https://github.com/koalaman/shellcheck