Refactor both Dockerfiles

[consideRatio]

General refactoring following best practices to the best of my knowledge.

• Added comments to guide the user for local development and comprehension on the purpose of various files
• Slimmed image size by doing some rm -rf on apt-get stuff
• Run apt-get upgrade to reduce known vuln
• Fixed a failure to propagate SIGTERM to python3 in the jupyterhub-ssh image, making it take 30 seconds to terminate in k8s
• Avoided a COPY . statement breaking build cache non-stop and risked copying sensitive content that could end up pushed to a registry
• Extracted a requirements.in and requirements.txt file to be managed by dependabot

[yuvipanda]

Thanks, @consideRatio!