Restructure docker build

[gkreitz]

An overhaul of how our docker images are built.

• Images depend on each other to the greatest extent I managed, to cut down build time and total size
• Use cache while building, to speed up subsequent builds
• Update maintainer to kattis' contact email, instead of an individual
• Build deb package outside of docker image, instead of extracting it
• Run verifyproblem on all examples when building docker images, to test the built deb
• Adds a problemtools/githubci image which I intend to use for our github actions (hopefully speeding things up, and allowing us to run verifyproblem on all examples in CI)