Frigg-worker

Fix
- Add requirements/base.txt to manifest.in

Fix
- Add x-frigg-worker-token header to hq requests

Feature
- Add support for secrets and env variables from task
- Add build specific environment variables

Fix
- Fix error handling of git clone

Feature
- Use docker image specified in task object from queue

Breaking
- This change will require the frigg-dispatcher endpoint to provide an image to use in docker.