I believe Appveyor meets your wishlist. It's known as a Windows build service but they recently added Linux support. We're actually moving away from them thoug

I believe Appveyor meets your wishlist. It's known as a Windows build service but they recently added Linux support. We're actually moving away from them though because they don't support build chains and artifact dependencies which is becoming an absolute requirement now. TeamCity is a much better fit for our workflow but Appveyor was very helpful for us to get started. Aside from that, the only real downside however is that they update the base images without warning which can be a real pain if that happens to break something in the middle of a sprint. Overall however I think the service is excellent, well designed and their support team are very responsive and knowledgeable.