Revert to ruby 2.5

[tbille]

Done

Downgrade to ruby 2.5 Disable renovate in docker

QA

• DOCKER_BUILDKIT=1 docker build --build-arg BUILD_ID=test --tag design.ubuntu.com .
• docker run -ti -p "8006:80" --env SECRET_KEY=secret_key design.ubuntu.com
• http://localhost:8006 play around

[webteam-app]

Starting demo at: https://design-ubuntu-com-canonical-web-and-design-pr-157.run.demo.haus/