Add custom backend host in container variable

[licebmi]

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Description

Add a BACKEND_HOST variable to be able to change the name of the backend upstream on the fronted.

This uses nginx container templating capabilities. This is not a feature of nginx as an application, but a script that is part of the official nginx container.

Screenshot (if UI-related)

To-Dos

backend

• [ ] Successful build npm run lint
• [ ] Successful test npm run test
• [ ] Database migration (if required)

frontend

• [X] Successful build npm run build

Issues Fixed or Closed

• Fixes #67