security: harden Docker images by enforcing non-root user execution

[RinZ27]

Currently, the application and client containers run as the root user by default. This
configuration poses a security risk, as any process breakout could lead to full container
compromise.

Technical Change:

• Implemented non-root user execution within Dockerfile (using user node) and client/Dockerfile
  (using user nginx).
• Adjusted directory permissions to ensure the applications can still operate correctly without
  elevated privileges.
• Properly handled PID file and cache directories for the Nginx process in the client container.

Following these Docker security best practices significantly reduces the attack surface and aligns
with the principle of least privilege.