Most major services offer different package deals, though you seem to want an on-demand service. Amazon offers exactly what you want: https://aws.amazon.com/getting-started/projects/set-up-ci-cd...

looking around at other services: Gitlab won't add it to gitlab.com hosted repositories, nor does bitbucket seem to offer auto-magically rescaling + on-demand. https://gitlab.com/gitlab-org/gitlab-ee/issues/3314 https://bitbucket.org/product/pricing?tab=host-in-the-cloud

However you could run gitlab in an automatically scaling AWS package (or google cloud, azure, any other service that offers elastic packages).