If you are a indie developer and looking for CI around the budget of $50. If your test suite is 10 minutes.
Buildkite is $15 per month and to be honest, you could run tests on your own MacBook Pro while boostraping your business :) Run on your Macbook, you dont need docker, your development environment already works.
For other products, your test suite scaling has a limit by their offering.
I know a friend who has few web servers and other 20 web servers that are crawlers. Those 20 crawlers is not constantly operating at 100%. Install a [buildkite-agent] and now they can act as test runners. Each Agent costs about 15-25MB of Memory, 0.2 - 0.5% (idle).