Get the environment variables to the template parsing

[teemuvesala]

Is your feature request related to a problem? Please describe.

In some cases we need external services where the Host-header must be correct. At the moment there isn't way to do that. By default the Varnish uses the same Host-header which it was called with. So that header must be different in different environments. It would simplify life a lot if I could use environment variables for that.

So we'd had at template:
set req.http.Host = "{{ .Env.CORRECT_EXT_HOST }}";

And at the Helm template I'd had environment variables defined correctly. This is not the only place where I'd use it. I've got some special requirements for the URL rewriting. This would help also at that.

Describe alternatives you've considered

Additional solution would be the script which pre-parses the template, but then we'd have too many levels of parsing. It would be error prone and waste of time to maintain.

[mittwald-machine]

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