Fix cryptography

Third time's the charm with Git branches...

This PR rewrites the Dockerfiles to use python:3.7-slim rather than the Alpine images so we can get pre-built .whls for the new Rust-based Python cryptography package.

See https://github.com/pyca/cryptography/issues/5771 for more details about the underlying issue. Since our Dockerfile used the python:alpine base image, there wasn't a prebuilt .whl for the new cryptography package, and there are a few hoops to jump through to install a working Rust compiler.

As an aside, Python + alpine images tend to be larger than Python + Debian-slim images because things rely on GCC.