Are you planing to use ytt (which is based on Starlark)?

https://get-ytt.io/

ytt is a templating tool that understands YAML structure
allowing you to focus on your data instead of how to properly escape it.

Info about Starlark:
https://docs.bazel.build/versions/master/skylark/language.html

Drone supports Starlark via plugins (see https://github.com/drone/drone-convert-starlark). There is no native support for Starlark. Please note that anyone can create a plugin which means you will be able to create a ytt plugin and share with the community.