docker: switch to python slim image and use pip for dependencies

This MR updates the Dockerfile so the terms-extraction image is based on the lightweight debian-buster-based python:2.7-slim-buster image.

This official python image runs the same debian version as the previous one, while being lighter and already including python.

The python executable set to use by this image takes precedence over the system's python, so packages are installed via pip. Packages are pinned to the versions available at https://packages.debian.org/buster so they remain the same as those installed via apt with the previous base image.

Image size is brought down to 390MB with this change from the previous 738MB