Use simpler base image on rspec jobs

Summary

While !667 (merged) introduced rspec job into our pipeline as of %11.8, the base image on that job is larger than required. The base image should be simpler.

as is

• Total duration: 10:13
• Duration for loading the base image: 01:48

https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/1137882404

to be

• Total duration: 10:30
• Duration for loading the base image: 01:48

https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/1152950589

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Part of gitlab-org/gitlab-build-images#70 (closed)

cf. #2639 (closed) for rubocop job