Changelog: Add dependencies of changelog_manager script to Docker image

Add Ruby & gems (rugged, activesupport) for scipts/changelog_manager.rb.

• Adds ruby to the installation of dependencies
• Adds needs for compiling rugged, installs gems, then purges build dependencies.

Also: added .dockerignore to reduce context sent with docker build, as it doesn't need anything from the repository at this time.

Allow changelog_manager stage to fail

Until we sort out permitting the CI pipeline to push back to master.

Relates to #343 (closed)