Exchange rate API not allowed by NGINX config

[mayrmartin]

• I have searched the issues of this repository and believe that this is not a duplicate.

Description 😯

In PR #1406 (closed) the endpoint to fetch the exchange rates was made dynamically. This leads to an error when the access point is not set to the default value as it is not marked in the NGINX config as an allowed host.
Furthermore, there is an issue regarding older releases.
There was a change regarding the endpoint where we fetch the currency per default. This change was made by changing the URL but not reflecting the changes in the NGINX config. This issue was reported on a Trubduget Sandbox which runs on 1.30.

Proposed ToDos:

• if the exchange URL should be dynamic, allow the domain in the NGINX config by making it also dynamic
• Evaluate how this issue impacts older versions and how big the impact is

[SamuelPull]

FYI nginx conf update has been done in https://github.com/openkfw/TruBudget/pull/1411/files

[SamuelPull]

Issue is twofold: nginx config doesn't combine multiple "add scp header" directives, but chooses the most restrictive one. Rule for additional forex api wasn't applied.
Frontend container was always using the predefined forex url, because config is fixed at build time.
What's missing is a default in env.js and its substition with configureServer.sh script.
Version 1.30 has hardcoded url that ECB retired this year, nothing that can be done about it.