Draft: Secure the frontend container image

!!! DO NOT MERGE YET !!!

This MR introduces a more secure base image of nginx that is unprivileged, and fixes some security vulnerabilities.

Depends on: https://code.europa.eu/oots/devops/oots-common-services-containers/-/merge_requests/24

Please Note: Breaking changes ahead...

This will introduce breaking changes, as the default port is changed to 8080 for security reasons.

We will probably have to update some manifests here:

• https://code.europa.eu/oots/common-services/cs-devops
• https://code.europa.eu/oots/devops/oots-common-services-containers
• https://code.europa.eu/oots/devops/oots-common-services-orchestration

From a functional point of view, the user would not know the difference, but our containers would be significantly more secure.