Looks like this is a policy-as-code tool in the vein of Terraform Cloud's Sentinel Policies, or more generally Open Policy Agent, but specifically targeted at k8s use cases.
We just released support for custom rules :) from interviewing our users, we decided to start with [0] JSON Schema as it is very easy to write rules using it and you do not have to learn rego.
Having said that, we might add OPA .rego support in the near future :)
What is the desired way for you to write custom policy rules?
From the custom rules overview https://hub.datree.io/custom-rules-overview , though it is docs WIP, I noticed these are defined as YAML/JSON somehow. That's a contrast to HashiCorp's Sentinel https://docs.hashicorp.com/sentinel/concepts/language and OPA's Rego https://www.openpolicyagent.org/docs/latest/policy-language/ . Is this an intentional design decision?