[IMP] doodba-copier-template: domains_prod and domains_test

[norlinhenrik]

Answering the copier questions, I was misleaded by the domain_prod and domain_test default value {} when a custom value should be a list.

I suggest default value null. When the user deletes the default value, a simple placeholder will show up.

[pedrobaeza]

I don't see the need of this, as there's already the same 3 lines below. Maybe we should add cert_resolver: letsencypt by default. cc @josep-tecnativa

[norlinhenrik]

I have removed duplicate lines.
The placeholder has the comment in the end, to show first the syntax of the value.

[norlinhenrik]

The inverseproxy_shared network was not created out of the box. So I've updated the documetation to say that the traefik yaml should be inside an inverseproxy directory.

@pedrobaeza @josep-tecnativa Is there anything you would like me to change in this PR?

[Tardo]

This was a change I made due to problems with "None" values. But if with "null" everything works correctly, please change the rest of the defaults to null... it would be the closest step to how it was before my changes: https://github.com/Tecnativa/doodba-copier-template/blame/9e48836230b19afc25103776f86077b3c5037b40/copier.yml

Docs: https://github.com/Tecnativa/doodba-copier-template/blob/main/docs/faq.md#when-upgrading-from-an-old-template-copier-fails-with-invalid-answer-none-how-to-update

** And yes, I made a mistake in considering those questions as dictionaries and not as a list of dictionaries.