Fixate Ruby and bundler versions on docker images

The applications Gemfile now specify both and after the upgrade they are different than the ones found at debian:unstable. This causes the service images build to fail: https://gitlab.com/interscity/interscity-platform/interscity-platform/-/jobs/280504020.

The ruby docker images are also debian based. So I do not expect this base image change to be a breaking one.

Part of #15 (closed)