Improve efficiency of running the most simple CI/CD builds

Description

Slack: https://gitlab.slack.com/archives/C0SFP840G/p1681376329482399

We are still using a ruby:3.1 image as a default one, even when someone does not specify an image: in their .gitlab-ci.yml at all.

It appears that we are not caching the image, because pulling it still takes 30+ seconds in a simple hello world build on GitLab.com:

# .gitlab-ci.yml

hello:
  script: echo "Hello World"

Proposal

We should cache the default image and other, most popular ones by default, to reduce the time to start script block execution on shared runners.

This may increase the speed of very simple builds by at least 50%.

/cc @tmaczukin @clefelhocz1 @nicolewilliams