Pre-compile Bootsnap cache inside gitlab-rails image

Relates to gitlab-org/charts/gitlab#1779 (closed)

Add a pre-compilation of Shopify/Bootsnap cache within the gitlab-rails container(s).

We perform the cache generation by running rails runner bogus, knowing that it will fail. This return is ignored, by the use of ; vs &&, and then we output the resulting size of the /srv/gitlab/tmp directories which should only error if generation failed entirely.

cc @stanhu

Currently depends on mainline release of Bootsnap after https://github.com/Shopify/bootsnap/pull/283 was merged. The needed change to the cache keying is only present within master at this time.

Needed for gitlab-org/charts/gitlab!1157 (merged)