Reduce size of built image
Reduce size of built image

- Switch to python:3.7-slim-buster as the base image, as Pipenv is
  targeting 3.7 (not 3.8) and we want a more stable base image
- Run build in multiple stages, so that we only need the compiler and
  developer tools during building.
- Use Pipenv to install in system mode, but redirect pip to run in user
  mode, and to install everything, even existing system files. This
  allows us to very easily copy the entire /root/.local folder to the
  runtime image.
- Install required uWSGI runtime libraries manually in the runtime
  image.
- Tests show this reduces the final image from 1.12GiB to 332MiB.

dgnorth/drift-base,dgnorth/drift-base