I want to setup our Continuous Integration with CircleCI. I want our GitHub to deploy our code to a central test org anytime a developer pushes changes to run all the Apex tests and make sure that nothing is broken so we can deploy to a packaging org. I have found some repositories online like this or this that have the configuration setup but my CircleCI builds are still failing. It seems to be simple enough but I must be missing a simple step. Can someone either share a working setup or a reliable source that can help it get setup please?
One more thing is that, unfortunately, we have not made the jump to DX yet. So it needs to work with pre-DX tools and orgs. Our current infrastructure is working with ANT and and-salesforce.jar.