Update runtime image from Ruby 2.7 to Ruby 3.0
Updates the base image from Ruby 2.7 to Ruby 3.0.
Also updates gitlab-styles from 6.0.0 to 6.1.0 to use TargetRubyVersion: 3.0
cf. gitlab-org/gitlab-styles!75
Close #24 (closed)
Edited by Takuya Noguchi