cryptography does not support 3.6 - docker build fails

Package version (if known): from repo - v1.0.0rc1 - 95feace

## Describe the bug
Docker backend build fails since cryptography does not support python 3.6.

## Steps to Reproduce
Build using `docker-compose -f docker-compose.full.yml up`

## Expected behavior
Build succeeds.

## Additional context
Fix: update docker/backend/Dockerfile to use python 3.7