Fixed environment variables references in Caddyfile.tmpl

[tk-nguyen]

Hello, I fixed some mistakes in the template Caddyfile. Environment variables should be reference as {$}, as described in https://caddyserver.com/docs/caddyfile/concepts#environment-variables.

Thanks!

[techknowlogick]

Thanks for this PR, this file isn't directly used by Caddy itself, but rather the linux tool envsub creates a file for Caddy.

If you are able to remove the dependence on envsub with this PR that would be appreciated.