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No CI? Gitlab and Bitbucket have built-in pipelines.

What's the point in GitHub creating yet-another-CI-tool? Travis and Circle CI have had good integrations for years and work very well.

Cost?

Circle CI works well on their free tier, so no, not cost.

If you're OK with one container, sure.

You can always purchase more plus open-source projects get four containers for free.

There is a whole section for CI services.