Let's assume the project is written in Ruby.
I found that for linting, Rubocop is a useful library because it not only lints (analyzes) the code but also formats it.
For testing, RSpec is a good choice, as it is widely used and has sub-libraries for different purposes.
Since Ruby is an interpreted language, there isn't a traditional compilation process, but Rake is a strong candidate for so-called "building." It can install dependencies, compile assets if needed, set up the configuration, and also implement linting and testing stages.
If we consider alternatives to Jenkins and GitHub Actions for setting up CI, the top three would likely be GitLab CI/CD, CircleCI, and Travis CI.
Since the team is small, a cloud-based environment would be more suitable. This way, there is no need for a person or even a team to maintain the environment, and a cloud-based option is affordable for a small team like ours. However, a small team doesn’t always mean there are no special requirements for the project, such as heavy processes or unusual needs. In such cases, a self-hosted option could be a better fit. So, apart from team size, the choice really depends on the project's specific requirements and needs.