Avoid to use Debian stretch (9.x) by using Debian buster (10.x) in ruby images

In 2018-05, @stanhu, in !98 (merged), introduced pinning of the base image of ruby (image: ruby:*) from arbitrary to -stretch (meaning jessie to stretch) at that time, when the latest stable of Debian was stretch, while ruby:2.3 and ruby:2.4 were based on jessie. Afterwards, we updated the relevant code in !320 (merged) and !355 (merged) by @stanhu and @ayufan .

The reason for !98 (merged) is described in #20 (closed) to pin OpenSSL version.

Keeping this legacy code is now causing the security problem on build and dev (Gitpod, for example).

References

• .gitlab-ci.yml on !98 (merged) merged: https://gitlab.com/gitlab-org/gitlab-build-images/-/blob/202de09d92679c3c9321ad2cdc299905721c8694/.gitlab-ci.yml
• Add Debian Buster and remove Jessie: docker-library/ruby#287
• Remove 2.4 (EOL): docker-library/ruby#315