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 (gitlab-ce#21626) with common modules pre-loaded.

Proposal

Links / references

• Meta speed issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/21624
• Create gitlab/kitchensink docker image