Pre-seed cache/image/whatever so first build is fast
Description
We support caching in between builds, but the first build of a new project always starts without a cache. We should speed that up somehow so onboard, and a user's first experience with GitLab CI is great. Some ideas:
- Provide pre-seeded caches for common scenarios, like Rails apps.
- Provide Docker images for common languages/frameworks, with pre-built modules.
- Kitchen sink image (#21626 (moved)) with common modules pre-loaded.