What is an actual use case for that? If you're providing a template, you surely control what's inside of a template and can just not use whatever is not supported by envsubst?

You can create a universal template with defaults and reduce amount of variables to passing into container. Something like resolver ${RESOLVER_URL:-127.0.0.11} valid=${RESOLVER_TTL:-30s} ipv6=off; in resolver.conf.template. Without this behaviour you have to keep all this defaults values in docker-compose\helm\other things that you use for configuring and templating your setups.

OK. But that means extra work for redoing templates in different environments that might have different defaults...

Anyway, that proposal would also mean we will have to build non-GNU envsubst for all supported platforms manually. I'm leaving it open, but it's definitely not on top of a things to implement.

I'm also interested in this capability. It can simplify image configuration a lot

I would love to have this feature as well! The idea is to have templates where most of the variables are optional, so you can run the application with little configuration, but still can be highly configurable if needed.