feat(devcontainer): Improve Rails initializer for devcontainer

[jjmata]

(from a Copilot PR review)

[nitpick] The session key is hardcoded as '_app_session'. Consider using Rails.application.config.session_options[:key] or a configurable value to maintain consistency with the application's existing session configuration.

    key: Rails.application.config.session_options[:key],

Originally posted by @Copilot in #95 (comment)