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

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:

Drone supports Starlark via plugins (see 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.