fix: update dev backend and frontend root folder

[mutambaraf]

Description

This PR updates the root folders for both backend and frontend in the development environment.

Motivation and Context

This change was necessary due to updates introduced in PR #1347. We now want to mount the validation folder at the root and the app inside another layer (e.g., frontend) to match the ../../ folder structure.

Changes

• The root folder for the backend in the development environment has been updated from /app to /app/backend in the Dockerfile.dev file.
• The root folder for the frontend in the development environment has been updated from /app to /app/frontend in the Dockerfile.dev file.

These changes ensure that the backend and frontend directories are correctly located in their respective root folders, enhancing the structure and readability of the project.

How Has This Been Tested?

Fixes Jira Issue

https://jira.esss.lu.se/browse/

Depends On

Tests included/Docs Updated?

• I have added tests to cover my changes.
• All relevant doc has been updated