Previously the Bootsnap cache was populated with `rake about`.  While
this loads the Rails environment, it does not load a significant
amount of the GitLab Rails code.

This commit adds a separate image layer to populate the Bootsnap
cache. It does the following:

1. Builds `gitlab-rails-{ce,ee}` image.
2. Uses docker-compose to spin up a PostgreSQL, Redis, and the Rails image to
start `gitlab-rails runner`.
3. Builds a `gitlab-rails-bootsnap-{ce,ee}` image that contains the
Bootsnap cache in `/srv/gitlab/bootsnap`.

Images such as `gitlab-webservice-ee` then use
`gitlab-rails-bootsnap-ee` instead of `gitlab-rails-ee`.

This improves the hit rate significantly. Setting the `BOOTSNAP_LOG`
environment variable shows that previously there were 11,567 and
11,553 "[Bootsnap] miss" messages in `webservice` and `sidekiq`,
respectively. After this change, the number dropped to 29 and 26,
respectively.

More importantly, the startup times dropped about 15 seconds a...