fix(frontend/Dockerfile): enhanced permissions for nginx.conf substitution

[tom-rm-meyer-ISST]

Description

When checking the #255 the nginx.conf substitution for rate limitting had problems due to permissions.

Tested together with @f-zimmer. Problem did not occure anymore but we wanted to make sure it works with 777 rights instead of 117.

Pre-review checks

Please ensure to do as many of the following checks as possible, before asking for committer review:

• DEPENDENCIES are up-to-date. Dash license tool. Committers can open IP issues for restricted libs.
• Copyright and license header are present on all affected files