use python:3.6.8 docker image
use python:3.6.8 docker image

Changes base python docker image to fix issue #218.
The current python docker image (3.6.9) has `dh key too small`
errors when trying to download files from the S3 backend.

Duke-GCB/DukeDSHandoverService,Duke-GCB/DukeDSHandoverService,Duke-GCB/DukeDSHandoverService