Add RUBY_PLATFORM variable to able to support JRUBY

[ryudoawaru]

This PR makes it possible to support JRuby by setting RUBY_PLATFORM variable.

[elct9620]

Prefer to use rules to change it to constraint the value in ruby or jruby

job:
  # ...
  rules:
   - if: $RUBY_PLATFORM == "jruby"
      variables:
          RUBY_IMAGE: jruby

But it may constraint user to use rules

Maybe migrating to GitLab Components will be better for security
https://docs.gitlab.com/ee/ci/components/

[elct9620]

https://docs.gitlab.com/ee/ci/yaml/index.html#specinputs

The spec:inputs seems to work correctly for non-components.

GitLab Component only work for same GitLab server or use mirror (Premium or higher)

[elct9620]

It should work:

include:
  remote: https://github.com/elct9620/ruby-gitlab-ci/raw/main/rails.yml
  inputs:
    platform: jruby

However, the docker, javascript, sentry reuse common.yml therefore we have to add inputs for all in the current design.

include:
  - remote: https://github.com/elct9620/ruby-gitlab-ci/raw/main/rails.yml
    inputs:
      platform: jruby
  - remote: https://github.com/elct9620/ruby-gitlab-ci/raw/main/docker.yml
    inputs:
      platform: jruby

To resolve it, we have to remove common.yml dependency on it and make each component standalone.

[elct9620]

Due to the platform being designed for Ruby. We may rename it to ruby_platform and then add js_platform to support deno or other runtimes.