Hi Sean - haven't looked at BitBucket specifically, but these days they all seem to use a yaml config file.
If you want something custom, look at the one I use in PharoLambda - it's using GitLab (which also has private repos), but I suspect BitBucket is probably similar.
I don't use SmalltalkCI, just a plain set of build steps, but that's also another option if you are building multiple platforms on your CI.
Tim
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