delphi_python image fails to build

[dfarrow0]

Attempting a fresh build of the delphi_python image now (since the last week or so) fails. The failure happens when attempting to pip install scipy because linalg dependencies can't be found.

The acute issue is that:

• python 3.9 was released last week
• the latest python:3 image from dockerhub has already updated to python 3.9
• numpy and scipy (probably others) haven't yet uploaded pre-built binaries for python 3.9
• numpy and scipy fall back to building from source
• the base image (debian buster) doesn't include build dependencies
• build fails (e.g. CI has been broken the last ~week delphi-epidata#248)

The more general issue is that, as a matter of philosophy, docker images should be specified precisely in order to guarantee reproducability. delphi_python was using a generic base image of python:3, which caused things to break when that image upgraded from 3.8 to 3.9.

Related to #24: the primary server is on python 3.8.2.

The fix is to specify a specific python version. Proposal: python:3.8-buster

Eventually we'll want to upgrade to 3.9, but that's not recommended due to need for additional dependencies and significantly longer build times.