These days I've been using the free tier of OCI (Oracle's cloud thingy) [1] for my personnal CI/CD and some random services that don't have any disponibility requirements. They give out 4vCPUs (ARM) / 24GB machines. I threw Drone CI [2] on it and I don't have to worry about any quotas. Albeit the block storage attached to these instances is nothing to write home about (60 IOPS/GB). As a side note, Oracle's IAM is especially shit.

[1]: https://www.oracle.com/cloud/free/

[2]: https://www.drone.io

I used to use CircleCI but migrated to GH Actions when that got released. One thing to note about GH Actions is if you have a Pro account, you get 3k build minutes per month.

Possibly a stupid question but can any of these public CI services support the following:

1) Build containers easily, for each commit?

2) Provide hypervisor access for custom VMs?

3) Secure support for 1/2 for possibly hostile PRs?

I'm currently using self-hosted gitlab runners for 1/2 but not 3 and interested in alternatives.

What's the carbon footprint for these services? Would be interesting to calculate how much would an average free tier usage pollute. Are there any available numbers from these products?

This seems like it would be hard to pin down, since it would vary widely from project to project. It would kind of be like asking 'what's the carbon footprint of a dev running a test?'. Well, that depends on the complexity of the code they're testing, what computer they're using, what language they're using, the climate of their workspace, and so on.