Using Python 3.8 Alpine-based as our base image for the default Docker build, as we're slowly trying to move away from Python 2.7. This increases the image size a little bit due to a new Rust dependency needed to have `cryptography` running in Alpine (see https://cryptography.io/en/latest/installation/#alpine).
This PR:
- Changes the base image to `python:3.8-alpine`.
- Adds dependencies needed to have `cryptography` running in Alpine.