I am trying to build a Caddy configuration that can cater to multiple environments (production, staging, local development).
To achieve this, I want to use environment variables for the base host names/urls, via the {env.*} placeholder. This works great in addresses, route matches and upstream reverse-proxy configuration.

Unfortunately, using the same approach does currently not work when using On-Demand-TLS with the ask property because the placeholder is not replaced at runtime:

2020/12/14 21:49:44.939	DEBUG	http.stdlib	http: TLS handshake error from [::1]:62116: parsing ask URL: parse "http://{env.API_HOST}/domain-check": invalid character "{" in host name

Here's the relevant part of the config:

{
    [...]

    # Configure On-Demand-TLS for user sites with custom domains
    # https://caddyserver.com/docs/automatic-https#on-demand-tls
    on_demand_tls {
        ask "http://{env.API_HOST}/domain-check"
        burst 25
        interval 2m
    }
}

https:// {
    [...]

    tls "example@mail.com" {
        protocols "tls1.2"
        on_demand
    }

    respond "Hello, World"
}