Linting container failing in CI

[conorsch]

Description

The lint CI checks are failing, due to apt errors reported from the Debian Jessie repos.

Steps to Reproduce

1. Observe CI failing here: https://circleci.com/gh/freedomofpress/securedrop/24949
2. Run docker run -it gcr.io/google-containers/python:2.7.11-slim apt-get update locally and observe same failure.

Expected Behavior

Apt lists are fetched without error, container build proceeds, linter runs.

Actual Behavior

Apt lists fail during fetch, container built halts, linter does not run, CI fails.

Comments

While this may be a transient error, Debian Jessie was marked EOL last year, so we should immediately move the container to a Stretch-based image to ensure continued updates.

[cji]

The easiest solution might be to just switch to python:2.7.16-slim-stretch from Docker Hub, which is just a minor version bump in Python but on a stretch based image.