[WIP] Completely rework the build process

[thesuperzapper]

This is a rework of the build process to be more efficient and work properly.

Highlights:

• Only use docker for environment, don't build inside the image.
• Move to Python3 only (Python 2 is being dropped by Airflow very soon)
• Fix issues with metadata not being correct. (E.g. airflow_env.sh never being used)
• Work out of a tmp folder, rather than polluting git root.
• Allow user to specify the distro they want with --dist.
• Fix licence headers
• Remove Debian 7 (EOL)

To consider

• We should probably enable --enable-optimisations when building python, but this does massively increase build times.