feat(docker): enhance Dockerfile with caching and comments

[obeone]

This update introduces several improvements to the Dockerfile:

• Add comments.
• Set the base image to 3.12 to ensure sgpt will build, even if a new incompatible version of Python is released.
• Add a .dockerignore file.
• Use Dockerfile syntax version 1.5 for enhanced features.
• Configure DEBIAN_FRONTEND to noninteractive to suppress prompts during package installation.
• Disable automatic clean-up of the apt cache to retain downloaded packages.
• Implement caching for apt and pip to improve build efficiency.
• Reorder instructions to optimize layers (don't run apt-get if sources have been modified).

These changes aim to secure and optimize the build process.