optimize Dockerfile

[kovaacs]

The current Dockerfile isn't set up optimally, so any code change results in the whole image to be rebuilt from scratch.

• Reduce Layers: Combined apt-get update, install, and cleanup into a single RUN command to minimize image layers.
• Leverage Docker Cache: Copied requirements.txt first to ensure Python dependencies are installed only when requirements.txt changes. This improves build times for subsequent builds when application code is updated but dependencies remain the same.
• Smaller Image Size: Used --no-install-recommends to install only necessary packages and cleaned up package cache (apt-get clean and rm -rf /var/lib/apt/lists/*).